Help with Computer, keeps shutting down

I know i probably be on another forum but i havent joined the others and im having problem with my pc. The anti virus wasnt up to date and i rarely use it but tonight i had to use it fo a short while and it keeps shutting down on me. Sometimes straight away and other times afetr a few minutes. one time long enough for me to type in the anti virus product key but not long enough to scan etc...
Any ideas if it is a virus what i can do????