Games Crashing my PC?

First post, Bear with me as I know next to Nothing on this.

Okay so here I am, Talking with some Halo CE/PC clan friends in an Xfire chat room.
Next, We decide We're all Bored so we decide to play some CoD4. I'm playing,
Then my screen Goes blank, I Hear my Fan go Full speed, Then My computer reboots, But It's stuck at "Starting Windows". I decide that a system restore is my only option, So I go with that. For about the next week, No problems.

Until..

Right, So my Brand-Spanking New Copy Of Halo 2 Arrives. Boy am I excited, I suspect that after MS Shut down Original XBL, The PC Community will recive a much needed Boost. I don't activate my copy yet, I just want to play some campaign First. I load up the armory, And just as you get out of the tram-thing with johnson, I hear Lots of Sound Glitches, And the same thing as before. It does it a few more times. Then I played some DiRT 2 about 20 minute's ago, It it does the same thing.
Now, I suspect It's only Games For Windows LIVE (GFWL) Games doing it, But then I remember that CoD4 did it. Now I'm afraid that if I play anymore games, And do this too much, That I will Seriously Screw My computer.

Did you recently OverClock your CPU/GPU?

No, What else could be the problem?

Which Antivirus do you use?

Norton 360.

Did recently encounter any problem with virus?

Well not any "Virus" And a full system scan just ran.