My old computer goes into a reboot loop.?

My old computer (which is now my grandmothers) got a BSOD last night. She didn't tell me about it so I don't know the error message it gave. But today when she tried to start up the computer it would get to the logo screen and just suddenly restart. When I try to reboot under safe mode, as it loads the drivers it gets to MUP.sys (i believe that's what it is....) and reboots. I have ran MEMTEST and nothing came up. I tried Windows Repair... nothing wrong. I am currently running MHDD on it to see if anything is wrong there. Any other ideas? (I don't want to reinstall windows if i can help it)