Well, for one thing you need to change that default workgroup name; that - I have discovered - can cause issues with sharing through XP Pro. I've never experienced this myself, as I have always used a unique name.
You don't actually share files - that's a misdescription on Microsoft's part - because you actually assign sharing to folders via the Browse system.
Can you confirm to me that you have run the Add a Network Place Wizard on each PC for every new shared network place? Or have you been doing this some other way? More precise details are required, please.
I'll post back tomorrow PM if possible.