Why a computer will keep corrupting OS?

Hi everyone. I am working on a computer that first had a bad HDD, so I replaced it. Now I installed a new one, installed Windows Vista 64-bit and after a week the OS started having problems to start up until it completely stop booting up.
I checked the memory sticks with memtest, checked the motherboard with pc-doctor and had no errors.
So far the only thing I doubt to cause the problem could be a bad PSU.
Any suggestions?. Thanks in advance.