computer lock up

I have an emachine T2692. It would no longer boot. I would get a power light, the cpu fan would run but I get no video or disk activity. I replace the psu with from another computer with the same results. I put my original psu back in and replaced the mobo/cpu with one from another computer and it booted up fine. This made me think either a bad mobo or cpu. I bought a new mobo and used my original cpu in it. I now have my original PC with a new mobo. Now, it sometimes behaves as when I first had problems. Sometimes it seems to boot fine with HD activity but it would hang when it tried to load sup.sys (WinXp). I thought maybe it was due to the change in the mobo. I have tried reloading WinXp from the restore CD and have tried to install Win2K. When trying to install them, the PC hangs or freezes at various spots (never the same place). I have read that a psu can cause a pc to hang or freeze.

Can a cpu cause this, a bad cpu, or something else I am overlooking? what should I try next?