Windows 7: Please help with auto exiting fullscreen.

This problem is that when I was playing games on steam called Alien Swarm in fullscreen and it exits its fullscreen off. I don't have that security called VBD or something.. Please help me. It bugges me like playing games and it exits it self. Isn't there any solution for it? I am really sick of that exiting problems. I thought it was actually my computer's video card's problem but I recently changed it but it still exits full screen.
Please help.



Thanks.