I went to check out a friend's PC...I don't know if it was booting at that point or not because the monitor wasn't working, and I didn't pay attention to what the CPU was doing. Brought it home, hooked it up to my peripherals, and it wouldn't boot. The power light is on, the HD light was on briefly, then nothing else. I opened it and tried starting it with various hardware disconnected (CD, DVD, floppy, PCI cards removed, etc.)...still nothing. I replaced the AGP graphics card with one from another PC, and it then started up. So I bought and installed a new graphics card, and it still worked fine with that.
Here's the problem...
I took it back to my friend, hooked it up to his peripherals, turned it on, and it wouldn't boot (same lights as before and no sound of HD spinning). I disconnected all peripherals in different combinations. Still no boot. So I brought it back home, hooked it up to my stuff again...no boot. The PS fan is running, as is the processor one. It's running Win XP.
What to try next?