For what it is worth in my case it was my non-windows firewall that was causing me the issue. As soon as I marked the connections as 'Trusted' in the firewalls on all three of my PCs they all saw each other and I am able to share files and devices.
I am having exactly the same issue. I have a XP workgroup containing two XP computers, this allows device and file sharing without issue. I have now purchased a netbook which has Windows 7. I have changed the worgroup name on the Win7 netbook to be the same as the XP workgroup. On the XP...