help!my laptop switches off automatically without any reason

hello!
plz help!my laptop-acer travelmate4150- switches off automatically.smtimes it happens when it gets warm.so i was thinking it was warming up too much.but smtimes it will works for hours and nothing happens and smtimes switches off even after 30 mins after ive turned it on for the 1st time.it occurs more often when im trying to buffer sm movies from the internet.i cant get to scan for viruses also since it switches off before scanning is completed.should i get a cooler or smth?ive saved all my data on my external USB drive.
please help!!thx!