Speaking of upgrading things, we installed MSSQL 2000 on a new server (Win2K3) and copied over our stuff from the old server.
The copying was less than flawless. Everything was busted. The main problem was that it seemed like the users were associated with the right databases, but they weren't given access to them. You couldn't grant access to those databases because MSSQL 2000 complained that there was already a user with the same name in that DB!
Google, saving my ass once again, pointed me to this page. Basically, when you copy or restore a database, it's possible that your users may be different or have different SIDs on the server. Thus, the users in the databases become orphaned. That page shows how to restore the association between users.