Black Screen starting.

I have a problem to start CF. When I press "Start" button, the x-trap runs good till 100% then the screen goes black and frozen (when it's supposed go to the log in username and password screen); i can't do anything to get out beside forced to reboot the PC. My system is AMD Phenom 9600 Quad Core, 8gb Ram and 1gb ATI Radeon HD 3870 X2. I don't have any Ati Virus, and also turn off Window Firewall, Window Defence, but the problem still there. Please help!