Evidently it wasn't working fine if you did a complete restore, which didn't work. Then you tried a new/different harddrive, and it's still not booting to xp. Something is causing this problem and that is why we keep telling you it could be bad ram. Bad ram causes all manner of problems. And this is why I suggested you run the memtest overnight.

Power supply problems don't usually refuse to let you load windows, they usually won't even let you turn the computer on.

Possible other problems could be bad IDE/SATA cable, depending on harddrive type.

The problem with you have has to be narrowed down. We can't just say, it's the harddrive or it's the ram or it's the psu.

If you have another psu around, you could try that too.