Is it asking for a username and password, or just a password?
If's it's Access wrkgrp login screen, copy system.mdw from your office install disks and use c:\WINNT\system32\WRKGADM.EXE to point it to the system.mdw file you took off of the disc.
But I bet this is something different, especially if it just asks you for a password, like a Word or Excel protected doc.