I was installing CRM 2011 on new set of machines and got the above error. The account that was being used had all the privileges required for doing the installation.
While searching for the solution found out one thread that mentioned that the error could be because the SQL Server Management Studio query window might be open and could be locking one of the system DB. It was hard to believe however I gave it a try, closed the Management Studio window on the SQL Server machine and retried and the installation was successful.
Update :- The next time i had the management studio closed but had a remote session open to the SQL Server machine, disconnecting the session resolved the issue.
Hope it helps.