Must be a very common problem and I have had it return again. However I seem to have tried most things including Microsft "5 ways to keep you PC clean" and Reg Cure and System Restore and De frag etc etc.
My computer freezes completely Crt,Alt,Delete won't work and the only way to get it back is to switch off, wait a min or two and start again.
The strangest thing is that there is absolutely no pattern. One day it will do it with 2 mins, the next not for 2 hours, the next 4 times a day the next not at all!