Everyone's already covered the basics. The only thing I'd like to add is that ever since I've been building gaming machines and Folding@Home rigs, Windows invariably falls over and dies.
Reinstalling Windows and getting back to where you were is a major pain.
I'm sure there are some free or shareware solutions out there, but I bought
Acronis True Image.
Once I get my system set up and completely happy, I burn a disk image that allows me to get right back to a working state much faster than reinstalling from scratch.
To me, it's well worth the money.