New RAM installed recently?
System fans running OK?
Installed any new hardware/software recently?
Check those in that order. RAM has this horrifying habit of coming up faulty, especially non-branded types (sorry, but it's a truth, at least in my experience)
The computer overheating is another, but should only occur after the system has been used for a while (depends on processor speed really)
finally, it could be the result of installing something (hard or software) new that Windows has taken a dislike to - try removing it, and see if that fixes it.