Access 2007 running Access 2000 database - user level security

Help Please!!

We have upgraded software to Access 2007, whilst we are able to open the
2000 database, all users and permissions have been lost - I have been able to
join the existing workgroup on some PCs, however I am unable to do it on
others, does anyone know how I might restore this?

I get a message saying "The security wizard cannot be run on a database when
you are both logged on as the Admin user and the wizard has previously run on
the database. To fix this problem, you may log on as another member of the
Admins group or import all objects to a new database."

Access has reverted to default users & groups - I am unable to log in as a
different member of admins. I have replicated the database but again I am
unable to join the workgroup used previously.

Any suggestions?

Thanks