My first suspect would be XP SP3. The primary updates of SP3 were to tighten up the Windows Firewall and tighten up security in general.
Many users with small peer to peer networks have had to start over with the configuration of their XP systems after installing SP3.
I myself would be more than a little concerned about having a Windows 98 system on any network that does not have a hardware firewall protecting it. Windows 98 still has security holes that will not be patched. You also have problems getting a good up to date anti-virus package on a W98 system.
Chas