My system about 3 years old

Dell XPS Gen3 pci expresss
windows xp pro
pent 4 3.2
2 g ddr2 533 mem
8800gt video card 2months old
sound blaster sound

Recently upgrade from 1 gig memeory to new 2 gigs of memeory 2 weeks ago.
Computer ran fine the day before but went to turn it on yesterday and it STARTS up fine for first second then all the fans kick on at high speed the power light turns YELLOW and that is as far as it goes fans at full speed and wont boot.
I have tried putting the old memory back and and no help. BLEW out the inside which was pretty clean. Removed the video card and put it back in along with sound card.
This has hapened before but turning it off and back always took care of it but had not happened in over a month.

Widows is up todate with all files.I would try to reset the cmos first. Turn off the computer, unplug the computer, OPEN the case and find the small battery on the motherboard. Make sure you ground yourself by TOUCHING a piece of metal from the case before taking the battery out. Leave it out for about 20 minuets. Push the power button a couple times also (I have heard there can be some stored power left in the caps?) Put the battery back and start the computer.  I doubt this will do anything, but it is a quick thing to try.

If I had to guess, I would say it is your power supply. I say that because I see you added a fairly power hungry graphics card plus more ram. You may be reaching the limits of your power supply.  How big and what kind of power supply is it? Should be listed on the side of it.