EMERGENCY: My laptop keeps shutting down randomly, how do I stop it?

I have windows movie maker 2.6 and windows live movie maker although windows movie maker 2.6 is preferable as i find it easier to use! I was wondering if anyone knows how to rip videos off youtube (virus free) =) and put them into windows movie maker 2.6 and edit them and stuff. any help would be gratefully recieved! =) lol xxx  unlock iphone 3g