The router can't be giving you the same ip address, if it did, nothing would work.
As for seeing each other, on the linux box make sure that either samba is configured to report to the w2k server (add it to the domain) or that nmbd is running in broadcast mode so that the w2k machine knows its there. Then make sure you have at least one browseable share configured in samba's configuration, that you have encrypted passwords enabled, and that you pick one of the following:
1) security=user - make sure that you use smbpasswd to create users for every user of the w2k box, and set their passwords to match the w2k password.
2) security=share - this will allow true anonymous access (to shares marked as such), since it wont require a username to browse. You will, however, have to mark each share you want password protected as owned by a particular user, who you then create with smbpasswd with whatever password.
Note that smbpasswd is completely seperate from the normal linux password system (in most cases).