Computer suddenly shuts down

Hey, is there anyone who can help me. While Im working,
after like 20 min. a pop up window comes out and tells me
that my computer needs to shut down! I cant close anyohter
application and I have to click OK. Or sometimes the pop up
window wont come out, it just suddenly ****s down. I dont
have any warning when I enter to Windows about a virus and
that I have 1 hr to work cause it will suddenly shut down
or restar.

If anyonw can help me, I would appreciate it.

Thanks

Chad