|I had this problem once.|
I would press keys such as "R" and Run... would open, or "N" and the Narrator would pop up. This was after installing MW2 from Steam.
All I did was a hard reset (restarted from reset switch), after the POST pressed F8 to go into the advanced startup options and started from last known good, this worked for me, because it was my first restart.
If you have already restarted this may or may not work for you, you could also try (if all else fails) booting up in Safe Mode go into device manager, find your keyboard, and update or roll-back driver, try switching USB ports, or even if you have one, try another keyboard.
I remember trying to find an answer to this on google before and couldnt find anything so hope you manage to sort it out because I know how much of a pain in the ass it can be!