|Almost anything could cause this. Temps, power, bad parts, bad software. |
You will have to either try to get a clue from some report or error log to take a guess in a starting place. Otherwise replace something and then move on till it gets fixed.
Try a live cd to see how stable it is. Make a Vista PE disk to help get close to your setup. A live linux may not be a good test but better than nothing.
1/3 of highway deaths are caused by drunks. The rest are by people who can't drive any better than a drunk.