Here’s a strange one - I’m running XP on an Athlon XP2000 with 256mb RAM, and have just changed the PSU and case. I turn the PC on and it starts to boot as normal then, halfway through the ‘initializing XP’ sequence the screen goes blank and, after a couple of seconds, the monitor turns off. The PC’s still running and the HDD light is still on so presumably it’s still initializing.. Turning the monitor back on makes no difference as there’s no signal going to it, I just get the same monitor test message that I get if I turn the monitor on without the PC. If I press reset the same thing happens.
I then give up and turn the PC off... a couple of hours later I find that it’s turned itself back on again all by itself! (I suppose it is Halloween...)
Is it possible that the new PSU is playing up? could try refitting the old one. Don't run XP so don't know if you can run msconfig to shut down all startup progs then reinstate them one at a time until the problem recurs.