At least now we know it's not just Internet Explorer causing the lock-ups.
How long does it take to freeze once it's been switched on? If you can open the case, can you see if all of the fans inside the case are spinning when the box is switched on? If not then something could be overheating.
Honestly, the possibilities are endless. If you can get to the event viewer it'll help alot to determine what's going on. You could be having a hard drive failure - if so, there should be some evidence in the event log.
Could also be a power supply problem, graphics card problem, memory problem, CPU problem, motherboard problem, etc, etc, etc. Any one of these might cause the symptonms you're seeing.
As for resetting any settings he's changed, well it depends what he might have changed. Anything that would cause the symptoms you're experiencing I would suspect would have been changed in the BIOS, not something you can do from within Windows. There's usually an option within the BIOS to load failsafe settings or something worded similar. You could try that and see if it makes a difference, but it's all a stab in the dark really.
When I'm presented with similar problems, I usually have some "spares" so I can do some troubleshooting, swapping out the key hardware components and see if any one of them (or a combination) is causing the problem. You might eventually need to take it to a PC repair shop, if you know of a reputable one in your area.[/quote]