Won't work with Vista

There are no particular drivers for any USB devices . They are all Plug and Play devices and will be automatically detected unless faulty or if you are using an older version of Windows ( 98 / 95 etc.).
XP /Vista / 7 will detect your device automatically. Try it on some other USB port on the back of the computer instead of the front ( where usually people tend to connect portable devices ) if its a desktop PC.

u cant do that

See the section about the hot fix first----- If you are using Windows XP: you need the following. Windows XP SP2 or higher Windows Media Player 10 or higher
Download and install the latest Windows Media Player if it's not installed.
If you are using Windows Vista: If the player is seen in My Computer when using MTP mode, but not showing in Windows Media Player. Download the hotfix from-->>> http://support.microsoft.com/kb/939211. Run the hotfix.
Refresh the players driver Connect the Sansa player to the PC The screen on your player should turn on, show 'Connected' and indicate the battery charging or full. If the player does not turn on try other USB ports or on another computer if one is readily available. Ensure you are connecting directly to the computer USB port and not to a USB hub. - If there is nothing showing on the screen and the player does show up in as a storage device in an explorer window, then the screen is just not working.
If you receive the error "USB Device Not Recognized" on the computer, the player is most likely failed. Additional troubleshooting is unlikely to correct the problem. Check under Device Manager 1. Right-click My Computer, and select Manage. 2. Click Device Manager on the left pane.
NOTE: Sansa MP3 Players can connect to your computer in MSC or MTP mode. Exceptions are Sansa Connect and Sansa Express which only use MTP mode. MTP mode is the recommended and default method for Sansa Players when used with Windows OS. The Sansa player will show up on Device Manager's right pane as: 1. USB Mass Storage device (if the player is set to MSC) 2. Portable Device (if the player is set to MTP mode)
If there is a yellow exclamation point (!) or question mark (?) beside the device, try to refresh the drivers. Refresh the drivers 1. Right-click the entry with the error mark, select Uninstall. 2. On the top menu, click Action then select Scan for hardware changes.
NOTE: If a device with yellow exclamation point (!) continually appears and disappears, then the player is most likely failed. Additional troubleshooting is unlikely to correct the problem.
Connect using MSC mode Connecting in MSC mode will help determine if there is a problem with MTP mode or with the Auto Detect mode feature of the player. You can use Forced MSC mode or switch the player settings to MSC.
Instructions to connect in Forced MSC mode are listed here: http://kb.sandisk.com/app/answers/detail/a_id/61
Instructions to switch player to MSC mode are listed here: http://kb.sandisk.com/app/answers/detail/a_id/207
If the player is detected in MSC mode but not when set to MTP mode, try to update or reinstall the Sansa firmware
NOTE: Sansa Express and Sansa Connect do not have MSC mode. For Sansa Connect you can try reinstalling the firmware using the Connect Recovery Mode.
Additional troubleshooting notes: If you have replaced the Sansa Player and you still have problems getting it to recognize in the computer, then you may need to try some additional troubleshooting. - Try connecting on another computer. - Try using a different USB cable - Try connecting to the back USB ports which may supply better power. - If only MTP mode is not working, try reinstalling Windows Media Player For Windows XP try also download and install the MTP Porting Kit

Try other USB ports on your computer. A USB port behind a PC usually has the most power output. Try a different cable if there's one available, to rule out a cable malfunction. Try another computer.  NEXT - Refresh the drivers STEP 1 - Connect the Sansa player to the PC STEP 2 - Check under Device Manager -> 1. Right-click My Computer, and select Manage. NOTE: The Sansa player will show up on Device Manager's right pane as: 1. USB Mass Storage device (if the player is set to MSC) 2. Portable Device (if the player is set to MTP mode.)
See this tip posted here on this site. This might be what is going on with your player.  http://www.fixya.com/support/r3774047-solve_mp3_player_connection_problem 
If there is a yellow exclamation point (!) or question mark (?) beside the device, try to refresh the drivers.  STEP 3 - Refresh the drivers 1. Right-click the entry with the error mark, select Uninstall. 2. On the top menu, click Action then select Scan for hardware changes. A brand new Sansa player NOT recognized by the computer when plugged in: This may be caused by one of the following: 1. Minimum system requirements were not met. 2. Corrupted drivers. 3. Computer restrictions. 4. Power issue.   To resolve this issue, try the following solutions: Verify if minimum operating system requirements are met.
Operating systems supported by Sansa players: Windows Vista and Windows XP SP2
Windows Media player supported by Sansa players: Version 10 and 11 Download and install the latest Windows Media Player if it's not installed. 
For MAC users, see Sansa player not recognized by a MAC OS Next Connect the device to the PC in MSC mode View/c200:  Turn on then press and hold the rewind button while plugging to the PC  See List of Sansa players with instructions on enabling MSC mode
If all this has failed - Refresh the drivers
STEP 1 - Connect the Sansa player to the PC STEP 2 - Check under Device Manager 1. Right-click My Computer, and select Manage. SOLUTION 4 - If that didn't work, and you are running Windows Vista, verify if you have Service Pack 1 installed.   If all that does not work - Manually add the hardware driver. STEP 1 - Connect the Sansa player in MTP mode STEP 2 - Manually install the device driver 1. Click Start select Control Panel then select Add Hardware. 2. Click Next to begin the Add Hardware Wizard. 3. Click Yes, I have already connected the hardware, then Next. 4. Scroll to the end of the list and select Add a new hardware device then click Next 5. Select Install the hardware that I manually select from a list (Advanced) then click Next 6. Select Portable Device or Windows Media Device from the list, then click Next 7. Select MTP device, then click Next. 8. Click Next to start loading the drivers. 9. Click Finish to close the wizard.
STEP 3 - Verify if the device is detected properly 1. Double-click My Computer. 2. Check if the device is showing up under Portable devices. SOLUTION 6 - Set user restrictions.
Some computers have user right restrictions that are set to disable access to new hardware. STEP 1 - Log-in as an Administrator STEP 2 - Edit restriction NOTE: Disconnect the Sansa player if connected. 1. Click Start then Run, 2. Type cmd, then the press Enter. 3. After the prompt, type Net localgroup Administrators local service /add then press Enter.
STEP 3 - Reconnect your device

open windows media player and put the files you want on your mp3 player in the library, then plug your mp3 player into the computer and turn it on. A page will come up on the computer and ask if you want to sync your device to the media library, click yes and it should load.

See the FAQ here - http://www.sansa.com/support/sansa_view/faqs

The most common reason for this problem is not having the latest Windows Media Player installed, or having a Windows OS version that is not supported by the Sansa player.
The Sansa players support Windows XP SP2 and Windows Vista and above. If your operating system version is supported by the Sansa player:
SOLUTION 1 – Download and install the latest Windows Media Player

SOLUTION 2 – If that didn’t help, and you are running Windows Vista, apply the following hotfix - KB940199
SOLUTION 3 – If that didn’t help, download and Install the MTP Porting Kit (click here)
SOLUTION 4 – If that didn’t help, try connecting the device to the PC using MSC mode

View/c200: Turn on & press and hold the rewind button while plugging into the PC

what you will need to o is to install the vista driver..Once you have the driver install the device should be working without any problems..
Follow the link below to obtain the driver for your device..Select your device model and operating system.....
http://www.sandisk.com/DriverDownload/index.asp

Thank you
Computerrep

Sansa c200 series – Computer/OS Not Recognizing
Connect the device to the PC in MSC Mode
View/c200: Turn on then press and hold the rewind button while plugging to the PC
If that fails, unplug the Sansa from the computer. Uninstall all the Sansa software in the add/remove programs section of Control Panel. Reinstall the clean version of the software from Sandisk’s website – make sure you get the files for the c200 series. Then replug the Sansa and try again. The root of the problem is that Sansa has one set of drivers and Windows has another. If you plug the mp3 player into the computer before installing the Sansa drivers, the Windows drivers will trump the Sansa’s.
Here is a helpful link from Sandisk on the problem with extra information on Vista operating systems:
http://kb.sandisk.com/cgi-bin/sandisk_en.cfg/php/enduser/std_adp.php?p_faqid=42&p_created=1221328944&p_sid=8VPwlRmj&p_brand=sansa&p_accessibility=0&p_redirect=&p_lva=&p_sp=cF9zcmNoPTEmcF9zb3J0X2J5PSZwX2dyaWRzb3J0PSZwX3Jvd19jbnQ9NzQsNzQmcF9wcm9kcz00JnBfY2F0cz0mcF9wdj0xLjQmcF9jdj0mcF9wYWdlPTEmcF9zZWFyY2hfdGV4dD1jMjAw&p_li=&p_topview=1

-Tha Mp3 Doctor

I have recently connected to a Sansa device. The device had to be powered on before it would be detected as a drive. Also it had a hold feature which suspended any I/O. Alternately, the Dell may not have all the usb ports connected to the motherboard. I suggest you plug the Sansa device into the same usb you were connecting the external drive. Beyond this, more information required. I hope it helps. Good Luck.

Open device manager under the Control Panel to view hardware and devices. There should be a tab under portable devices that recognizes the Sansa c250. If their is a yellow exclamation point, then most likely the device has stopped working. Right click on it, and uninstall the device. Unplug your Sansa, wait a couple seconds, then plug it back in. Let it do its thing. If this does not solve your problem, then you should deffinately call the manufacturer.