Quote Originally Posted by SKYMTL View Post
There is a 50 / 50 chance of it happening with the ATI drivers. I have no idea why but one way or another, you should reboot in order to ensure everything is working properly.
Windows Vista and 7 let you install from scratch and also replace on the fly any kind of video driver, reloading it in the process.

If you get prompted for a reboot then maybe it's some other driver from the package, like south/northbridge stuff in case you use them, HDMI sound driver or the like.