Random system freezes!

Hello, I've been having some random system freezes once in a while on my computer. The "intensity" of the freezes varies, sometimes it's only my firefox and the task manager that does not want to start up (and when they do after 10 minutes they're not responding), other times it's everything, including simply clicking the start menu, that takes half a minute to work. When I close down the computer I get the message that explorer.exe (the logoff sound) does not answer.
I do not know what causes this, and I've run the sfc scan and memory diagnostics both which say that there's no errors. I've done extensive malware/virus scans which also found nothing. I've run a registry cleaner but that doesn't seem to have solved the problem either. Any suggestions to what might cause this? I can't do a system restore as I do not remember when the problem first started.

@Jack: Using bullguard security for anti-virus/firewall. Will try to check for drivers then, hope that works. But I'm unable to replicate the error at will, so will have to see how it goes the next few days if I experience it again.

@Lordbob: I don't really know how to stop OCing it, the manufacturers from which I bought the PC OCed it for me.

Before I do that I'm just wondering if this could be causing problems with my system: I had some trouble installing a driver at some point which caused it to freeze during the install so I had to close it down. I never managed to get it to install correctly, but I guess it could have managed to change some stuff before it froze which could cause problems? It was a driver for a wireless network card.