Unable to access Win98 shares from XP

I have a WinXP Pro laptop that connects to a network to access shares
on a Win98SE server. When connected direct to the local network all is
great, the basic non-password protected Win98 shares can be accessed
from the XP laptop directly without providing any usernames/passwords.

However when the XP laptop connects to the network through a router, a
dialog box is launched requesting a username and password to access
the Win98 server.

What username/password should be used? Can this be disabled?

Win98 machines connected through the router can connect directly to
shares on the server without this problem.
All PC's are in a simple workgroup configuration, running TCP/IP.

Any help would be greatly appreciated,

Kind Regards

Richard