Networking XP, W2000 and W98

Have 4 PCs in home network: 1 with XP Home edition, 1 with Windows 2000, 2 with Windows 98se.
IP are fixed, ICS is done via Proxy software and is working fine.
But no filesharing is possible: PCs don’t even see the others in the workgroup. Tried several protocols, even NETBEUI. What’s wrong? Can you help ?!