Problem: Freezes then normal function

Hello everyone,

Long time lurker here, first time posting. I have a very strange and disturbing problem. I'm running Windows 7 Home Premium 64 bit. Almost 3-4 weeks ago my computer started freezing shortly after starting windows up. The computer would start all the programs, run for 5-10 minutes and suddenly freezes - mouse, keyboard light... everything.

After a hard-reset or turning off, the problem might persist for one or two more times, at different times, and then the PC would work normally for as long as I want. I could do anything, load huge files, play CPU- and GPU-intensive games, or just leave it running without doing anything, and it would not freeze at all. Only those two or three times when I first turn it on.

I'm assuming this awkward problem doesn't relate to hardware issues, as this problem would definitely persist throughout the working time of the PC. This is why I'm thinking it might be some software issue. I'm loath to install Windows 7 again, so any advice from the amazing community here would be much appreciated.

Thank you!