Vista - Request for a Simple Step-By-Step Guide to Networking Vista and XP

So far, I have not found a clear answer to this question anywhere, so I will
try to request one here:

I have four machines in my home network connected through a Belkin router.
Two machines are wired, two wireless. Three are running XP, one is now
running Vista RC1. The Vista machine is wired. When all four machines were
on the workgroup "COSTELLO," they could all see/share with each other.

Since installing Vista RC1, I have not been able to access the Vista machine
from any of the others. I can access the XP machines from the Vista machine,
but not vice-versa.

I have shared the Vista's folders; I have enabled full access for
"Everyone," I have even tried turning off the firewall. It simply will not
work.

I could go into a longer list of the things I have tried, but in most of
these posts the topic is getting muddier because we are each using different
permutations to try to get this working. If possible, I would like to find
out -- step-by-step -- how a Vista machine should be set up in order to share
with XP machines on a home network.

Cheers.