Win 7 Cannot Access XP PC

I have a network (LAN) of several machines at home. The other day I
tried to access OLDPC with XP from my DENPC with Win7. I got an access
error for //OLDPC from . Both are on MILKWAY group. I somehow managed to
fix matters from the XP machine, so that I could access it from Win7. A
few days later I was getting the access error from Win7. Whatever I did
on the XP machine doesn't work any longer. The ethernet cables seem
properly connected (visually)

I am not really able to verify the groupname on the Win7 PC. Under the
Control Panel Network Sharing Icon it shows HomeGroup: Joined. Joined
opened shows four choices. None show me the group name. One is Leave the
Homegroup, but the choices do not seem to give any insight into what the
Homegroup name is. The WinPC does not show it sharing with anyone else
on the LAN, which is OK by me.

I've found Win7 in this respect on other features. That is, to find
something to display, one often has to dig into choices that eventually
reveal what you are looking for, rather than making it real obvious.

Comments?