I've found in the past, at least when restoring databases, that the backup will keep the user record in the sysusers table that is local to the database, however, the UID in the user database does not match the UID in the master database. When Enterprise Manager shows you the users in the database, your username will not appear (I believe because the join between the two sysuser tables is failing). When you go to re-add your user, to checks to see that the username does not exist. Unfortunatly, the user name does exist. An easy way to fix this is to delete the user record from the user database sysuser table. You'll have to change the server settings to allow user modifications to be made directly to the system catalogs though.