Impossible to tell from a distance.

It seems a rather old machine. It might well be the useful part of its lifecycle has passed. That's the hardware point of view. It's not impossible.
If it's an old and trusted machine I don't believe in updating drivers. So the software point of view is malware on the machine. If none can be found (by other scanners than AVG) it's somewhat less likely, but not impossible either.

- If you look in the Event Viewer afterwards, does it show a problem? see http://support.microsoft.com/kb/308427
- Does it hang in such a way that even Task Manager can't be called? It's nice to be able to use ctrl-alt-del and look at the running processes.
- Is it reproducible (when he runs program A and does X) or random.

What I should do if no clues come up:
- Cleaning the vents (to eliminate heat related problems): http://mobileoffice.about.com/od/usingyourlaptop/a/cleanlaptop.htm
- If that doesn't help: system recovery back to factory conditions, to eliminate all software issues (lots of work, because you need to reapply SP2 - better do SP3 immediately) and reinstall all applications and restore all data.
- If that doesn't help either: discard the machine, or find someone for a cheap repair attempt (that's unlikely).

Kees