The problem is that the user in the database is an "orphan". This means
that there is no login id or password associated with the user. This is true
even if there is a login id that matches the user, since there is a GUID (called a
SID in Microsoft-speak) that has to match as well.
This used to be a pain to fix, but currently (SQL Server 2000, SP3) there
is a stored procedure that does the heavy lifting.
All of these instructions should be done as a database admin, with the
restored database selected.
First, make sure that this is the problem. This will lists the orphaned users:
EXEC sp_change_users_login 'Report'
If you already have a login id and password for this user, fix it by doing:
EXEC sp_change_users_login 'Auto_Fix', 'user'
If you want to create a new login id and password for this user, fix it by doing:
EXEC sp_change_users_login 'Auto_Fix', 'user', 'login', 'password'