Compter will not start up , it only sits at the Dell Logo with the bar not fully loaded

Press the Command and R keys at the same time when the grey screen appears and hold them until you see the Apple logo. A small loading bar will appear under the logo. Sit tight as your system boots into Recovery Mode. Click the Utilities tab in the top menu bar, select Terminal, type resetpassword and press Enter

Try contacting dell, they should be able to help you.
Here is a link that will explain what the diagnostic lights mean. This is for all Dell Desktop PC's.
http://support.dell.com/support/edocs/systems/Op745/en/UG_en/trouble.htm
I hope this helps.
Good luck and have a pleasent day.

load dell diagnostics and check your hard drive, one of the options when the dell logo is on the screen.

this is often related to a possible motherboard problem. Try these steps: turn it off, remove the battery, and AC adapter, then press and hold the power button for about 5 to 10 seconds. Once done, reconnect at least the adapter , turn on the laptop and as soon as the Dell logo shows up, tap F2 a few times to enter System Setup (BIOS screen). From there, go to the Maintenance category, and select Load Defaults, press Enter. Press ESC, then select Save/Exit, restart the computer and see how it goes.

Immediately after seeing the Dell logo, keep tapping the 'F8' key,
until you see a "menu" (white text over a black background)
giving "startup" options for Windows XP.

Select "Safe Mode" and see if you can start in that mode.
Then, use "System Restore" to go "back" in time, to the most-recent checkpoint (date/time) where your computer _DID_ work.

Restore to that point, and restart.

press the Fn Key and the power button see if the diags will run. You may have a hard drive problem or an issue with the operating system

You go into Boot Menu, and set the - first boot device to CDROM.
The computer will then read what is in the optical drive, first.

Computer off, press the Power On button. Watch closely for the blue dell logo.
Looking at the bottom of the monitor screen when the blue Dell logo comes up, you will see - F2 = Setup.
At the top right of the screen you will see F12 = Boot Menu.

This screen goes by F-A-S-T!
As SOON as you see the blue Dell logo press the F12 key.

If you miss this small window of opportunity, just wait for Windows to fully load, then click on Start, Turn Off Computer, and Restart.

If Windows is not loading, then press the CTRL, ALT, and DELete keys at the SAME TIME. (Not shouting)
This will restart your computer so you can try again.

Upon entering Boot Menu you cannot use your mouse anymore, you have to use the keyboard.

The Boot Device Menu has the devices listed, and each device has a number next to it.

At the bottom of the menu, type the number that is next to CDROM, then press the Enter key.

This is Dell Support, and the main page for the Dell Dimension 4700 Service Manual,

http://support.dell.com/support/edocs/systems/dim4700/sm/index.htm

Left-click on - Entering System Setup

Should you have any questions concerning this, simply state so in a Comment.

As soon as you turn your computer on, and see the Dell logo, start tapping the F2 key.
If you miss this window of opportunity, wait until Windows has fully loaded, then restart your computer. Try again.

(The Dell logo comes up pretty fast. If you miss entering BIOS Setup, {System Setup is Dell's term}, then as soon as you press the Power On button, start tapping on the F2 key. Tap in 2 second intervals)

http://support.dell.com/support/edocs/systems/xlob/dtg/en/systemse.htm#wp1211139

Tap the f7 or f8 key, when you turn it on, it should go into safe mode, Go into start programs, accesories, tools, and restore, select restore, select a date closest to today and let it go through the restore pocess, if it suceeds and starts up, rub a good virus program right away, it sounds like you picked up a virus or trojen, just be patient and it shoul be ok, if it won't go into safe mode, let me know, that will require more work. And before you do anything, unplug, remove the battery for a few minutes and replace, try it and if it still won't run, do the above, thanks ,GYM

Try Cleaning Ram Fingers with rubber and re inserting could be a bit of corrosion or moisture/fluff this often works