Problem with Sql Server 2005

Hello Friends,

                        I am using windows vista Home Premium, I installed Visual studio 2005 and Sql Server  2005. But when I try attach database I am getting the following error:

                       The server principal "User-PC\User" is not able to acces the database "model" under the current security context. ( Microsoft Sql Server, Error:916)

                       And when I try to create a new database i am getting the following error:

                &n ...