I recently formatted Windows XP on my Dell Dimension 3000 after taking out the HDD and copying its contents to another computer. The formatted computer had been running fine for 1-2 weeks afterwards. My dad was on the computer today checking his email when he said some black horizontal lines started moving across the screen for about 15 seconds before the computer shut off. Ever since, the computer will turn on for anywhere from 3-60 seconds, then shuts off without ever loading Windows. The green lights on the front of the computer turn on and at least one fan runs, but the computer doesn't make much noise other than that. How do I know for sure if it's the PSU or MOTHERBOARD or what?First, check you monitor cable , then eliminate your monitor.....hook up another monitor and see what happens.

If you have a video card, remove it and RESEAT it.

Is your heatsink firmly seated on the processor and flush to the motherboard......

Reseat memory.

Swap in a known good PSU of equal or higher wattage.

Please post the PCs' specs as well...
Here is your documentation ....

http://support.dell.com/support/edocs/systems/dim3000/en/SM/adtshoot.htm#wp1043338I tried a different monitor and cables, no change. The heatsink is firmly against the motherboard. The video card feels firmly connected, but I can still try to reseat it.

My network lights indicate memory failure. I tried reseating both CARDS, then using one at a time. With one memory card, the network lights indicate memory failure again, but using the other alone only powers up the computer for a split second with only the "C" network LIGHT glowing green (A, B, D are off), then turns off. From this, is it safe to assume my RAM is the problem, or should I continue checking the video card and PSU?Rule #1 is to never assume.

I find it strange your memory went bad. Maybe run Memtest on it to be sure....the power supply could be making the memory look bad......Broni or Patio .......your experience please......It's probably the PSU; recently had a Dimension 3000 do exactly the same thing; found the 3.3V output was dead. You can test the outputs of the PSU.

http://wiki.xtronics.com/index.php/ATX_Pinout
or
http://www.hardwarebook.info/ATX_Power_Supply

Remove AC line cord.
Remove all connections to mobo and drives.
Connect pin 14 to any ground of the connector (13 or 15 are good).
Connect AC line cord.
Test for voltages
I would agree that it sounds like your power supplyNot coincidentally, my brother has an identical Dell. I just switched in his PSU and the formerly messed-up computer started normally. Thanks for the advice everybody. Now to order a new PSU...Just out of curiousity, anyone know where to get the cheapest but reliable replacement? Never shopped for a new PSU before. Any tips?newegg