I have Win 7 setup as Work Network I am not joined to any home group, I turned on discovery, have turned on shared, turned on public sharing, turned off passwords. From Win XP Pro I can not access public shared files in Win 7 from My Network Places. I can ping the Win 7 machine I can also add network place manually to the public folders on the Win 7 machine. When I view workgroup computers from My Network Places. I get an error of workgroup not accessible. You may not have permission to use this resource.
Honestly, I've always found XP networking to be sketchy at best. Have you tried \\\\hostname yet and see if you can access shares that way? That's what I ended up doing even with an all XP lan.