Reinstalling Xp Was Never So Hard

[Sephiroth]

*bashes head into the wall*, ok, I'm better now, I think

I'm not sure what's wrong with my uncle's Athlon..... specs:

Mobo: MSI K7N2 Delta (Nvidia Nforce2 Chipset)
CPU: AMD Athlon 2500+
RAM: 2x 256mb Kingston Hyperx PC2700 DDR
HDD: 1 older WD 10gb drive
CDROM: an older HP 32x cdrom
PSU: it's 300 watts... I don't know the make of it, it's not labeled anywhere on the thing
Video: An AGP Geforce 4 MX440

Everything else is integrated into the motherboard..... now, here's the problem, for a few months now it's been randomly shutting itself off, and you have to flip the PSU off then back on for it to turn back on. I went to reinstall Windows on it tonight, and it's turned into a nightmare, at the same point in XP (and 2K) setup it crashes every single time, either when it asks you what drive to install to or shortly there after when it's copying files.

I've tried changing the video card, hdd, cdrom, and ram with the ones in my P4, all of which I know are fine, and none of the above fixed the problem. So, I'm down to the motherboard, cpu, and power supply. I fairly sure it's not the motherboard, I ran a few diagnostic tools on it which all said that it was fine. The BIOS reports the power supply's voltages are for the most part ok, +3.3 = 3.32, +12 = 11.85, -12 = -11.7, + 5 - +5.05 , -5 = -5.05, give or take a few tenths in either direction on all the rails. I don't have another power supply here to test with, and I don't feel like the hassle of taking the psu out of my p4 and putting it in the athlon for testing right now.

Out of desperation, I took the hdd out of the athlon, threw it in my box, restored a ghost image to it, put it back in the athlon, and windows boots up fine, but the box cuts itself off when you try to login. I've tried a different power cable, with and without a surge protector in the middle between it and the wall outlet. The only other option is the CPU, but it's temps are fine for an athlon with a stock heatsink and no case fans, 46C at idle. At this point, I'm definatly leaning towards the power supply, but is there anything I could possibly be missing?