Do you have anything sitting close to your monitor, or contacting the screen in any way? The touch screen might be thinking it is being touched.

--Andrew

I'm noticing the same problem with my WoW on the same machine. It seems the mouse behaves this way more in the lower area of the screen: it unclicks me from both the game and internet browsers as well when I try and click the links while they're displayed towards the screen bottom. Clicking anywhere above the middle works just fine. I'm thinking I might have to rearrange my unkeyed buttons to the side bars and get used to click there until there's a solution to the problem. I wish there was one though, it drives me insanely mad when you're trying to do something and can't get to it even with a key since it just de-highlighted your window.