Sometimes, when you detach a database or when you restoring a database from backup, users that are attached to that database get orphaned or left behind. When you go to reconnect or restore that database, and assign it users, SQL Server will complain that a user already exists. Usually this means you have an orphaned user. Since this wasn't my fix I have to give kudos to my boss for the script. So here it is:

sp_change_users_login @action = 'REPORT'
--sp_change_users_login @action = 'AUTO_FIX', @UserNamePattern = '<Enter User Name Here>'

Run the first line in Query Analyzer. This will give you a list of the users that are orphaned. Then, comment out the first line and un-comment the second. Make sure to put the user name that is orphaned on the second line, and run the script. SQL Server will report that it fixed one user. You should now be able to connect to your database with that user.