The 3,4 Diagnostic leds that are showing on your tower reference a memory problem. If you reference your computer's manual

< ADMIN NOTE: Broken link has been removed from this post by Dell> and click on ther "owner's manual" not the service manual. then navigate to the Diagnostic LEDs section. you will see it's a memory issue. Try this:

 

reseat your memory stick(s). then see if you get a different set of diag. leds. if they didn't change then strip the computer down to just the motherboard, power supply, processor. Then see if you get a memory beep code. If there isn't a beep code then it's a bad MB & CPU.

 

Work with that and see what happens. Hope this helps

 

 

 

 

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You'd be surprised to see how much info you can find from www.support.dell.com. I.E. guides for removing/installing parts, drivers, manuals for all specification needs, KB articles, etc. So before you post try searching the site to find the answer you are looking for.