You may know some of or all of this but here are the settings that have to match in order for the two to connect and share resources.
Workgroup=Must match on both PCs.
Client=Client for Microsoft Networks
Protocol=TCP/IP with matching IP address/subnet
Sharing=Is this enabled and devices shared?
Firewall=Is the firewall blocking access?
Connection=Are the correct cables being used.
Is the wireless router set to DHCP so that IP addresses are automatically given to the PCs. Run winipcfg on the 98 box. In the ethernet adapter slot make sure its on the nic and not a ppp adapter. Note the IP address and subnet are.
Run cmd on the XP box. Type IP config and note the address and subnet.
Try pinging the IP address of the other system. If they cannot ping each other they're not connected.
Don't forget that the windows 98 login and password must be added to the XP users list.
The win98 resources must be first shared before it will show over the network.
Finally check the firewall to make sure its not stoping the two from connecting.