Well, when you run the game full screen and go to the desktop, do you let it load the desktop before going into the game again? If you try to go back to the game, its still running, then what your computer is trying to do is work harder to load that program up, the visuals and everything, catch up to where you should be because it is still running, which is really stressing your processor.
If it is happening when you hit window key then go back into the game right away, I would either get a better processor(Dual core if it is not already) because it could be overloading, trying to do too much, or just wait a bit, when you go to the desktop, then wait a while till its smooth and running, then click WoW on the task bar, only once, and wait till it loads.. I play a lot of games and tend to chat on MSN and play at the same time so I always play Window'd mode to refrain from overloading the computer.
Many times my computer will just, freeze up, or at least the program itself, rather then a restart, but computers are all different. I'd suggest that (if this is a frequent event) run it in WIndow'd mode at full resolution(Not the highest setting but big enough to fit your screen. You will have a task bar, but you can do other things easier, and it shouldnt restart this way.
Other then that you could take that key out of the keyboard(haha) or just try and refrain from hitting it. One other suggestion is, instead of clicking on WoW on the taskbar, wait a bit and hold ALT and press TAB, you should see the task-changer come up, keep holding alt, and every time you press TAB it goes over one program. When running a game and pressing this it brings you to the desktop, just like pressing the windows key, doing it again goes back to last program, this is a quick and easy way to switch between multiple tasks, rather then clicking, if you hate going from keyboard to mouse. Hope my suggestions help.