OS won’t boot.

I was attempting to fix an old computer for a friend, i figured that it was just loaded with spyware/viruses and i could wipe it and re-load the OS. No windows, or live os will load onto it, XP runs and acts like it’s going to work, and then when i reboot the computer it goes through the main screen, and reboots. Same with Vista. It still has a partition on it, which i assume is XP, and it won’t let me delete it. I tried to run Slackware on the computer, and after the kernel uncompresses it reboots, same with knoppix. I thought maybe the power supply needed replaced, but i don’t want to do that for nothing.

Any ideas?