Lars,
I had this exact same problem a few months ago. Out of no where my computer would just freeze or even die completley without warning. I tried to reinstall the OS (XP Pro) thinking that was the problem, although 3/4 through the install the install hung as well. This really freaked me out because I had just replaced my HD last summer and now I though I was going to have to replace it again.

Well I opened up the case to check on the HD hookup and I happened to notice that the CPU cooling fins and fan were completely clogged with dust. The fan would not turn and the CPU was just overheating. one blast from my air can and I was back in business.

I think you need to look at all the variables when troubleshooting issues like this because it could be something involving your HW...

Just something to think about,

~ Knute