A lot of folks had this problem in AH1. It only happened to people who played the game in the same resoultion as their desktop. The fix there was to change the desktop resolution to less than that of the game.
For me, I upped game resolution to 1600x1200 and dropped desktop to 1024x768. The problem went away. Later I set them both back to 1280x1024 and the problem did not reccur and I havent been able to repeat it since.