A PC won't show if firewalled or not sharing a printer or folder. I read your post twice and see no mention about the basics. Also that firewall can be in the (un-named) router.
I have three PCs on a workgroup, all running WIN XP Pro. Two of then are wireless. The hardwire PC is connected to a router which of course is connected to the internet. the wireless PCs connect to the router to connect to the internet.
The problem I'm having is I can't see any PC on the workgroup for file sharing. I configured each PC with WIN network config, and sett the same setting for all three.
What am I doing wrong?