Easiest way to find a solution would be to try one of each computers with the connection tester.
RPC unavailable:
This is caused by the windows firewall blocking access or a third party firewall.
You can check on the affected client if the firewall is on by typing "netsh firewall show state"
If the XP firewall is ok, check for any third party firewalls.
Access denied:
The account performing the scan does not have the necessary permissions on the client.
Check with the connection tester first for any problems.