XPS 420 Freezes Up

I have an XPS 420 which I had used for over a year with no problem. It started freezing up. Ctrl-Alt-Del didn't work. The system time didn't change. I'm running Vista and considered upgrading to Windows 7, but was afraid it might freeze during the update. I couldn't find a pattern for what may be cause.

 I removed software, one package at a time, according to date installed. I bought a registry check package and ran it. I studied the Internet for similar problems.

I went through the Dell diagnostics and the result was that the hard drive needed to be replaced. However, I saw somewhere in the community discussion that heat may be a cause. I realized then my problems had started about the time I moved to a new house. When I moved, I had a place where the computer could go under the desk instead of on top.

I pulled the computer out five inches from the wall and three inches from the cabinet it was up against. That was four days ago and it hasn't frozen up yet, while prior to that it froze up two to six times per day.