My computer recently got zapped by a power OUTAGE. I got the Blue Screen of Death, so I wound up FORMATTING the HD and reinstalling Windows XP. Now, it either boots only to the Windows Logo screen, or to the screen that says it shut down improperly and to choose the mode of boot (Safe, etc.,). I reformatted and reinstalled three times, and got the same result each time. I don't know if this is a problem with the hard drive, or is it with the BIOS? It is a Dell 2400. Plain vanilla.Your power supply may have taken one for the team....It will STILL operate but the voltages may be out of whack.

See if you can detect any burnt electrical odor from the fan opening of the PS......Ideally, swap in a known good PS and see how it boots.If it was a power surge, it may be your motherboard.

Does it beep?My experience is that failed mother boards only give the system beep error about 1/3 of the time.Sounds like a bios or power supply problem.
Replacing them would be your options and one thing to add
if you do choose to go that route or even if you get a new pc
is to get a good surge PROTECTOR