Every time I use steam, any game, it shuts down after about 20 minutes. This has happened to me for over a year and I stopped playing a lot some time ago because of it. But now that COD4 MW2 has come out I started again

. I have checked with numerous softwares to check my computers heat and everything is on about 50 celsius... What happens is that it shuts off immedietly without notice and restarts. My videocard driver are up to date and this PC was built my the local shop. I also play WoW and it hasn't shut down while I was playing that for over 6 months ago.
I am illiterate when it comes to computers; I don't know what a bios is

so please do keep it simple.