The guy above proved my point, the one I always make. Computers don't mess themselves up, people do that to computers. Just use it properly and you won't even need to reformat. I've had this comp for eight months now and it was never formatted by me, not even after I brought it home. It has no hidden partitions or crap software, as I quickly and efficiently got rid of it. I barely even run AdAware anymore as I never need to, and when I do, it always comes up as completely clean.
It's about common sense, not formatting every month. Who the hell wants to waste all that time doing that? Not me, that's for sure.