my computer keeps turning off.

ok. i'm only 13, i don't know much about computers, but at the same time i know alot.
heres my problem. when i save or use windows movie maker, my computer will turn off without any warning. from researching, its possible that the fan isn't working and the computers overheating. i need to use WMM because i need to be able to finish making some videos/save them. its REALLY important. somebody told me its a virus. also, i'd like to bring up the fact that when i scan the computer for viruses, it turns off, i'm prety shure its overheating. i once got the message that its protecting my files, but that happend only once.

if somebody can help, please, tell me whats going on and how to fix it.