As markjoy suggests, the most likely culprit is a teenage boy.
My 13 year old has trashed every computer he has near to, including two laptops which actually belonged to him. The last one was a £750 Sony Vaio which slowed down to the point of being unusable and eventually would not even boot up; this happened twice and it's going back to Sony "under warranty" (it should not stop booting, obviously).
I don't think there is any way to prevent this. Windoze is too complicated and there are too many trojans around. I put some AV software on the Vaio and it found 13 nasty trojans. Most teenagers instantly to go any website, any link whatosever, without the slightest thought of what it might do. The "smarter" ones even download "new and improved" drivers for various bits on the motherboard; that usually trashes the computer completely.
The only practical way out of this is a backup of essential data, then a reformat, preferably using the recovery partition.
If the problem continues after that, it most likely a hardware failure. It's possible, for example, for a duff USB controller to use up 99.9% of CPU time.