Having sharing allowed and all the other required share/network settings
correct in Vista and XP machines I could not access Vista shared
foldews/printers from XP machines, however vice-versa I could. Now, after
three days I found the solution:

My Visata machine came with McAfee antivirus preloaded. Both the Windows and
McAfee firewalls were on. As soon as I disabled the McAfee firewall the
network sharing worked as intended!

Doing a little follo-up research, I then turned off Windows firewall and
re-enabled the McAfee firewall, as well as changed the security level setting
in McAfee. At this point I was back to the "no-access" problem so I disabled
McAfee firewall and re-enabled Windows firewall. The network worked like a
carm once again. Now I assume there is an exception check box like "Allow
network folder/pinter sharing" in McAfee's firewall like there is in windows
firewall if you are wanting to use McAfee's firewall instead. Also, this may
be the same issue if you have some other another firewall enabled.