I have installed server 2008 in my pc and configured the DC then I created a user and accounts. I am trying to login to the DC using user account nut but I am unable to loging to DC.
By default a user will not be able to login to the console of a domain controller. For security reasons you generally do not what users to have physical access to a DC or they can do evil things.
If you really think this is necessary follow the procedure described here To permit users to log on locally to a domain controller.