try pressing ALT + ESC
ALT + F4
alternatively you could try CTRL+ALT+DEL, This opens Windows Task Manager this will show all active windows on the Applications screen. choose the window you want to close and then click on End Task at the bottom.
THEN WAIT A FEW SECONDS FOR A MESSAGE TO APPEAR SAYING SOMETHING LIKE, "the window" is not responding, you can then choose End Now, Wiat, or Cancel ( i think ) Choose End Now, the window should then close...
hope this helps