What are the diagnostic lights showing in the back?
R/Don ID4400XP Home SP2Bios A06P4 2.8 GHz 400 FSB 1024MB RAMBFG 6200 OC 256 MB AGP
Might be the motherboard then. Strip the motherboard down. Disconnect all cards, drive cables etc. If the sys boots then start adding a device one at time until the problem returns. That would indicate the source of the problem. If it doesnt boot at all then it's ur mobo and ur going to need one.
Replace the rear fan if you have another 4 pin that will fit, my sister-in-laws 4700 did the same thing.
Dell Tech replaced everything, MB, CPU, RAM and Power Supply and the same thing happened again
Then a more experienced Dell Tech advised to replace the $3 fan and it worked.