SQL Server Permission Issues - SQL 2008 R2

I have a SQL 2000 MSDE database that I detatched from the SQL 2000 server and attached to a SQL 2008 R2 server.  After doing so, I was able to change all ODBC connections on the 20 workstations that use the SQL server application....I did need to add the "Domain Users" user group to the permission group of this database in order to do so (database is set is MIXED MODE).

NOW, however, for new installs of the application that user SQL server, even after I make changes to the ODBC connection string, I am unable to connect the application to the database.  The reason, it seems, is that the client frontend is trying to connect as a named user in the SQL database ... this user is still in the database, of course, since I just disconnected and reconnected the database from SQL 2000 to SQL 2008, BUT I do not see the user in the MASTER database, and I suspect this may be the issue.....

I do not know the password of this user, so I cannot just recreate the user in the MASTER database, or I would and this would probably fix the issue.  Any insight would be appreciated....I still have full access to the MASTER database on the SQL 2000 server, so I can do backups and restores to the SQL 2008 server, however I am reluctant to do this as the SQL 2008 server serves several other databases and applications.

Thanks!