What could be causing my friends computer to crash?

I am just round at a friends house trying to see what is wrong with the computer.

The problem is that after about 5 minutes of booting the system, it freezes up.

I have checked the most likely suspects, bios configuration, hardware conflicts, viruses, registry errors, and so on.

Everything seems to be ok, and it is not crashing at the moment, but has been crashing quite a lot.

The crash always happens a few minutes after booting the system, and seems to be fine so long as it does not crash in the first few minutes.

Any ideas?

Thanks.