have u recenetly installed any program like a antivirus etc?.
also u can go to start > run command prompt as adminstrator and type chkdsk /f /r
or try doing a repair with the windows disk
OR
when the pc boots , keep pressing F8, select repair computer ( top option i believe ) > system recovery options > select keyboard input ( uk , us etc ) > type username and password IF prompted > and i believe there is startup repair?