Computer issues...

Ok, I guess i'll begin where it started. I turned off my pc after reacquiring my network signal on my modem since it was out, and upon bootup it automatically rebooted once it started loading windows. Nothing fixed it. Last known good configuration, safe mode, nothing worked. Repair wouldnt work via the Windows XP disk, it would reboot once it got to the console screen. I swapped out hdds, memory, cdroms, took out my video card, basically down to the main pieces of the pc: mobo, processor, psu. I don't know exactly what the issue is. I think it's a 50/50 chance that it's the onboard IDE controller or the PSU. One thing I know is that it detects the drives fine upon starting up. Not entirely sure whether it is the IDE controller because of that. Any ideas? Computer is unusable, can't even install XP on another drive. I'm currently on my brothers pc. Sad