Stressing Me Out! Did I Mess This Up?

Ok so I go over to this guys house (he knows I'm not hardware expert) and he is having problems with bios during a backup program. While I'm browsing looking for the bios update (which I prolly wasn't going to do anyway) the monitor just shuts off. The computer is still running because when I push the power button I can hear windows shutting down. Long story short I decide to try a new power supply (his was only 300 W running 2 hard drives with other things) just to see what it would do. Well I tried another hard drive and it didn't change anything so I put his back in.

Then the hd with the XP OS wouldn't boot. I tried manually selecting the hard drive from the boot list and it would automattically revert to another drive that has 2000 on it. Also, when I looked at the hard drive it wasn't listed as Samsung Blah blah blah it was more like Sam[su3039483j.aaaa.
What could cause this?

I unplugged the working hd to force it to go into the XP drive and it booted fine. Then plugged the other drive back in and restarted. The name was correct again and the OS booted fine.

I leave.

I get a call about 15 min ago about how he tried to restart and it wont boot and the name is messed up.

I'm really getting a headache over this please help me out.