There are a number of possibilities.
Most likely its the Windows user access control (UAC) feature. Try right-clicking the SSMS icon, and then click Run as Administrator. That will provide your admin credentials. If that works, add your domain login as a SQL Server login. Then you won't need
to use your administrator credentials in the future.
If you think you already have a SQL Server login, possibly added during setup, then login with the sa account and look at the SQL Server error log. The error log has more information than is returned to the failed login attempt. Lood for the state mentioned
in the error. Then check out the error topic in Books online for the state description.
Rick Byham, Microsoft, SQL Server Books Online, Implies no warranty