Problem with something constantly pressing down....

Okay, this is the strangest problem I've ever seen. I have this problem where sometimes, in certain situations, my computer will continuously press the down arrow key and I can't do anything about it. These situations are rare, but when they occur it's extremely annoying. It's only happened to me in two areas so far. The main menu of a game called Lumines, not actually while I play the game and in Open Office when I try to redefine keyboard shortcuts.

At first it didn't happen in Lumines, then it did, now at this moment it doesn't anymore, I can play the game just fine. But now it's happening in Open Office. Restarting the computer and leaving the keyboard unplugged doesn't help, and when I was having the problem with Lumines, I reinstalled windows and it kept doing the same thing. I don't really want to reinstall windows just to not have it work again on Open Office, since I figure this is the same problem. Any insight on this predicament?