I did mine and Anna's the way Richard says, and once I'd changed the Belkin device for another brand, it all worked fine.
The two commands that helped me were IPCONFIG /ALL which gave the IP address of each machine on the network, and PING to check that they were seeing each other. Once that was known, the rest was easy.
I turned off the firewalls while I set it all up, and then turned them back on afterwards. One (the Windows one) caused problems and got shut down. Zone Alarm seems to be fine, although probably not needed since the modem/routers have their own hardware firewall.
File and disk sharing under XP is very easy. I had a bit more bother with a Win 2000 machine (sharing it from other PCs), but that became an advantage since that's now the backup machine, and I like the extra security.