Do you by chance use Vista/Win7 and alt-tabbed out of the game before?
Vista/Win7 tends to have a sticky ALT-Key. If you Switched focus by pushing ALT-TAB and then return to the game, each keypress is altered with the ALT-Key. While this might not be harmful for movement-keys, it's pretty obvious for pressing the number-shortcuts.
Just press ALT again, once you are back in the game and Vista/Win7 will register it's no longer pushed.
i have the same problem and im using Win 7. I actually found a solution in my frustration one night lol. I just hit the left alt and right alt key one after the other like 3 time and it resets it haha. crude, not permanent, but requires little work