adding WinXp pc to Win98 network

I have a network of 6 Win98 pcs. One of them, a central pc, provides a
shared database and an email service (vpop3). I am replacing them with new
WinXP pcs.
They are all set up with share-level access control.

My intention is to set up alongside an old PC a new one with the same
capabilities so that it can quietly slip into its place. Last of all I would
deal with the central PC hosting the email VPOP3 service and any other
central services I find.

I have set up the first PC, networked it, and got internet and email
working. However when using it, while I can see the central PC, access VPOP3
via the browser, get email, ping it etc I cannot see its file shares. The PC
appears in 'my network places' but attempting to open it I get the message
".... is not accessible. You might not have permissions...".

I have tried accessing it via net use and a unc name and share but failed.

I can see the file shares on all the other members of the workgroup, and the
other old PCs can see the shares on the central pc. The old pcs can also see
the file shares on the new windows xp pc.

The WinXP pc uses Simple File Sharing.
I have created a user with the same name and password on the WinXP and
Win98 crentral PC - a long shot!

I have given the router which provides DHCP and NAT a cursory look but
don't see anything there.

What is it that I cannot see - either on the win98 side or on the WinXP
side? Your advice would be much appreciated!