Windows De-selected For No Reason

Basically, what seems to be happening is when I'm using a program or something, like, for example, Google Chrome, the window will randomly be deselected or deactivated. It's like someone clicked on another window. Normally, this isn't a huge deal. I just click on the window and keep typing until it happens again (it usually happens two or three times then stops for a while); It's just annoying. But the real issue is when I'm playing a full-screen game, where it's constantly minimized. Not only does it take a good minute or two for my slow computer to pull the window back up, but this also sometimes crashes games, like Team Fortress 2.I haven't been able to play any games for a while.