My computer is having a problem

[Genius]

I'm thinking stickykeys, but the mouse wouldn't stop responding if that were the case.  I suppose it could treat it as though the window had been switched, but I don't think things would still move.  Can't really say what the case is for sure.  The beep really sounds like stickykeys, so I'm thinking Bluaki has the right idea.  Seriously, everyone has to turn that off at some point or another.

Next time it happens, try pressing alt+tab.  See if that switches you out of the game window.  If that works, then you should see whatever it is that caused it.