Possible PSU Problem?

[Eddage]

Ok, so about 6 months ago I decided to put some of the old computer parts I had lying around to use and made a computer for my sister. However about a month ago it started freezing and I couldn't for the life of me figure out why. The CPU wasn't getting particularly hot and running memtest didn't find anything wrong with the ram. I tried replacing certain components but all that led to was a major dent in my wallet, yet still more freezing.

 

I had a similar problem before (this thread), however none of the answers seemed to solve the problem back then and now it has reared its ugly head again.

 

The other day I tried changing the PSU with another one I had and at first it seemed to solve the problem. As before it would freeze within a couple of minutes of loading the computer it would now last a good few hours, I could even do some brief web browsing/music listening before it packed up. Due to this I thought that maybe it was a simple PSU error and if I bought a new, better one it would solve all my troubles. However due to already wasting money replacing parts that didn't need replacing I thought I would take the PSU out of my computer and test it with that first. Sadly the computer froze straight away with the new PSU which not only took me back to square one but brought a whole other problem to the playing field as well. After putting the PSU back into my computer and turning it on nothing appeared on the screen, it wasn't sending a signal to the monitor. This morning I tried again, this time using the monitor in the study and it seemed to be working, however my computer then decided to freeze just as it had finished logging on.

 

This pissed me right off, as this had never happened before on my computer and led me to believe that something in the other computer screwed up my PSU when I put it in. I have now unplugged my computer and don't wanna try turning it on again just in case whatever seemingly fucked my PSU can spread to other components in my computer!

 

Is it possible that there is something in my sisters computer that messed up my PSU? Its like some sort of virus got transferred to the PSU as soon as I put it in, if so is there any way to fix it or any way to fix my sisters computer?!

 

Cheers in advance for any help!