Could be one of a million things.

The only time recently that my system would lock up on me so badly like that was because of the Cydoor adware which was very buggy. After running Lavasoft's Ad-Aware and cleaning my system of adware and spyware, my random lockups went away.

I'm not saying that's your problem, I'm just saying the last time that happened to me, it was Cydoor's fault.