That sucks, if you reinstall steam, before you delete your old steam go to your czero folder and make a copy of the config.cfg file, like on your desktop or something, this saves all your binds and stuff, so after you reinstall you can move it back in there and have all your settings (except your video settings).
... when it reboots, does it blue screen for a second or anything, or just screen goes blank, then you see it starting up again? If its not the game itself it may be ram or power supply.
Also you can open up your case, and look at your motherboard, you will see a few dozen little round silver topped things sticking up that look like little batteries. They should be flat on the top, if you see any that the top is swollen, like the top is rounded, thats bad, and if you see any brown gunk coming out of any, just as bad, (the are capacitors ) and if you have a single one that is swollen, the board/computer is trash.
Here are some examples of regular vs swollen capacitors (the arrow is pointing to the bad ones, notice the others are flat on top)
