User account problem with cmd box

Hi there, I am hoping someone here can help. I have a desktop running windows 7 and I have set up 3 user accounts - mine the main one as administrator, my son & guest. When I try to launch the main one, I get the cmd box up and cannot exit it - when I put in the command exit it just goes to a black screen and stays there. Yet I can go on as a guest and all seems to work fine, but...