Problems Using BUILTINUsers For Connecting Unpriveleged Windows Accounts

In a previous thread, I got some help from you all in figuring out how to deploy a per-machine desktop application and database so that it would be available to all Windows accounts on the machine. Basically the advice was to create an SQL user for the database with the BUILTINUsers login which had the necessary privileges for connection.I have tried this approach, but I am still having problems with connections for unpriveleged users. I have given the database user the role of db_Owner as well as other roles, but it seems to make little difference. I have been trying the deployment on several machines running XP, and I get several different responses. But primarily I get the "CREATE DATABASE permission denied in database 'master'" error. All of these computers seem to have the same database user level permissions, but obviously there must be differences somewhere. That worries me from a deployment standpoint.

I don't want to make any changes at the server level login properties for Builtinusers, but I have experimented with changing these, and I've found that giving sysadmin privileges to BuiltinUsers works great. But I doubt that anybody would be very happy with that solution.

I'm confused about the "Create Database" message, because I'm not explicitly creating any database at application startup time. It also seems like this might be an attachment problem, but I'm not sure about anything. Is there really an attachment and detachment of the Express database every time the application is run?

So my bottom line questions are:
Which database level privileges do I need to give to my SQL user with BuiltinUsers login so that any Windows account can connect and write to the database?
Can I accomplish this goal without changing any server level privileges for the BuiltinUsers login.

Thank you.