Castle Wolfenstein enemy territory problem

Kinda weird problem with this, I downloaded it fine and the instalation went O.K, when I go to play the game it loads up, plays the little movie with the logo’s, shows the menu where you put seetings/name etc, and freezes. In the background you see clouds moving, but the keyboard and the mouse are both frozen their, I end up having to restart the computer. Any idea’s to what might cause this?