When running test on my Dell Vostro 1500, error code 0146 shows

That error code means that your hard drive is faulty or failed.
Try starting up in Safe Mode (using F8 key) ans see if that is ok.
If you need detailed steps, Microsoft has a page at http://answers.microsoft.com/en-us/windows/forum/windows_vista-system/error-code-2000-0146-unit-1dst-dell-laptop/8595604b-8cba-4ab8-b886-fad0725eb7cc

source/original from dell community

Welcome to Dell Community.
Let me first appreciate the fact that you have tried the steps yourself to isolate the problem.
Error code: 2000-0146 (DST Log contains previous errors) refers to previous errors in the hard drive which might not really be a hardware failure. In order to confirm this, I recommend you to run a custom test on the Hard drive which takes longer time than the Pre-boot System Assessment which you have already performed on the computer but it will give the accurate result.
Once the Pre-boot System Assessment is completed, the computer will give you an option to "Run the remaining memory test", please select "No". Then the computer will launch the 32-bit Diagnostics from the utility partition (if available).
After the diagnostics is launched, in the main menu, please click on "Custom Test", then select the "Hard drive" and click on "Run Tests". If the test completes without any error then the hard drive is fine and PC Restore can be performed to restore the computer back to the factory condition (which will result in data loss).
Please refer to the following article for more information on running the Dell 32-bit Diagnostics Utility: http://dell.to/MNJv4B
If you have any further questions feel free to reply.

This is indicative of a hard drive problem:
A user may report they receive an 0141, 0142, 0146 or a 0151 HDD (Hard Disk Drive) error on their Dell Desktop or Laptop system.

The Hard Drive may still continue to work but there may be other issues that could manifest themselves as an intermittent failure to boot to Windows, a slow system or noise from the Hard Drive.

If possible, before carrying out any troubleshooting steps on the Hard Drive, please make sure you have a verified working and up to date backup of your data

You could run the built-in diagnostic tool to determine specific HD problems, using this procedure:
To run the Enhanced Pre-boot System Assessment (ePSA) diagnostics, perform the steps below:

1. Restart the computer.
2. As the computer boots, press <F12> when the Dell Splash Screen appears.
3. At the One Time Boot menu, press the <Down> arrow key to highlight Diagnostics and press <enter>
4. A graphical menu listing all discovered devices in the system is now displayed.
5. Select Hard Drive and click Run Tests to start diagnostics.

methinks your drive could use a fitness check

click here for one such utility

One bad lead can cause a computer to continue on a cycle or to shutdown or fail to detect your hard drive

Test all leads that attach to your hard drive including electrical extensions,IDE,SATA

the leads from your ((motherboard to your hard drive)) make sure they have a secure connection and are not faulty or just replace them there probably old and faulty
make sure all leads that are attached to your drives dvd\cd have secure connections and are not faulty even the electic extensions or just replace them they are probably faulty a computer needs its connections to have an end so any faulty leads will end up with a computer error hope this helps
don

error stands for hard disk failure.contact service center

Code 0146 is hard drive read/write failure.

Replace the hard disk drive

2000-0146 = Hard Drive failure

Error Code 0146 refers to failure to find or load NTLDR files, needed to run your operating system. Place your Restore disk in the DVD drive, power up the computer, hold down the F12 key till boot screen shows up. Boot from DVD and run Repair to restore these files.

Hi there.
0146 error is DST Log contains previous error(s).
0148 error is IDE ERROR: status (x) (s)

This could be a problem with the BIOS, restart the PC and hit F2 as it starts up, this will take you into the bios, reset the bios to default setting, press F10, save and exit the bios, the PC will restart so perform another check and see if the same error code return.

Good luck