Thing is it could be many different things. OS got messed up and not loading correctly. Motherboard messed up. Memory gone bad. Hard drive gone bad. Fan/cooling not cooling unit enough.

I guess if I was troubleshooting it - I would first try to bootup with XP disk and reload XP (assuming XP is loaded)

After that, if you have extra equipment - try swapping out the hard drive for a different one.... same with memory.

it is just hard telling. I would hope for just a software (operating system reload), but it is hard to say.