When you restore a Microsoft SQL Server database on a different machine, you cannot access the database until you fix the permissions.
The problem is that the user in the database is an "orphan". This means that there is no login id or password associated with the user.
All of these instructions can be done via SQL Server Management Studio, with the restored database selected.
If you you want to remove the user from the database, fix it by doing the query below:
DROP USER user_name
This will drop the user from the database so that you will no longer receive the orphaned user message.