Windows 8 black screen after log on screen

after starting my comp. when it reaches the log on screen..... it shows my account is signing in and after that black screen with a cursor appears.... nothing else.... i have to sign out and then sign it again to get my screen visible and usable.... (or even restart again) please help me guys......

mine was doing that when i had Avast installed, but once i uninstall Avast and just use the build in AV never got that black screen again.