View Workgroup Computers can't see anything

[jap99]

I have 3 PCs connected via a D-Link 504T ADSL Router (wired). Internet connection from all 3 is fine.
Upstairs PC (running XP Pro) can see the other 2 PCs. The downstairs PCs (one running XP Pro and one running Windows ME) cannot see anyone, not even themselves. When I 'View Workgroup Computers' from either of these I get the message:
"Mshome is not accessible. You might not have permission to use this network resource. Contact the adminitrator of this server to find out if you have access permissions. The network path was not found".

Pinging the upstairs PC from either downstairs PC works fine.
Any ideas would be appreciated.

 

[DTracy]

1) Do you have file and print sharing running on all three?

2) Where is the router, upstairs on the XP?

David.

 

[jap99]

File and Print sharing is running on all 3.
The Router is downstairs attached to all 3 PCs by network cables. It is not attached to one particular PC.

Enable Netbios over TCP/IP is checked on all PCs, and Windows firewall has been disabled on the XP PCs. Both PCs are running Bullguard Antivirus/Firewall with same settings on both.
I'm not too worried about the Windows ME PC but I would like the downstairs XP PC to be able to see the upstairs one.