Virus or something else?

My friend's PC crashed a few days ago and it was due to corrupt windows system files. Now I am not very experienced in this area and neither is my friend. We thought that it was either a hard drive failure (we thought this was unlikely), a virus, or another type of hardware failure. Well my friend went out and bought a new hard drive thinking that if it was either of the first two that his problem would be solved. He also had to buy windows 7 as his PC did not come with installation discs. After he installed the hard drive and reinstalled the OS he found that the problem was still there. Now this has happened over a period of about a week and my friend doesn't know what it is. I have come to the conclusion that we do not have enough experience in this area of computers and that we could fix this problem faster if we went to the internet for help. Oh and please excuse my grammatical errors if there are any I stayed up late trying to help him fix his PC.