http://technet.microsoft.com/en-us/library/bb727037.aspx

Have a read of the above link re Vista File and Printer Sharing.

How to give Permissions are there, too.

If you are running Norton, Trend Micro, McAfee, etc’s Firewall, check its
settings to make sure it allows file and printer sharing..

1st thing to do is make sure that the Workgroup Name of ALL the computers is
the SAME.

In Vista Network and Sharing:

Network Discovery: ON (So it can see the other computers)

Network set to Private (Public is for hotspots, airports, etc)

File Sharing: ON

Public Folder Sharing: ON (Vista’s Public Folder is the same as XP’s Shared
Docs)

Password Protected: OFF (unless you want to set up identical usernames and
passwords on ALL computers in your Network) If you have it ON, you will be
asked for a username and password when you try to access a Vista computer
from an XP computer.

Also, run the XP’s Home Network File and Printer sharing Wizard.

Has anyone got a solution to this yet? My network developed the same
problem after installing SP1 on 2 Vista Business machines, prior to that
- no problems. Microsoft patch KB922120 installed on my 3 XP machines
allows all computers to show up on the "Vista full map" within "Network
& Sharing Centre", but not in the "View computers and devices". The XP
ones can't see the Vista ones and the Vista ones can't see any others
and only the Vista one attached to the printer can print. I have been
through many help screens including some that came up in the same search
that lead to this site, turned all the firewalls (AVG & Windows) off,
but all to no avail. All access the internet through a router attached
to a network hub without any problem.

OK Guys, panic over. Disabling AVG 8.0 firewall on all computers solves
the problem. Had only disabled it on the Vista ones earlier and that did
not work either for the Vista ones or for the XP ones. Seems AVG 8.0
firewall does not allow networking, but Windows firewall seems to be ok.