[Rob M]

Sounds like a firewall/permissions problem to me.

Try disabling all firewalls on both machines (if they're running XP they probably have the Windows firewall running by default).

Next make sure that both machines have an identical account set up on them (same user name and password).

Then try the Pings again.

Another thing you can try is to go to Start > Search

Computers or People

A computer on the Network

Type the name of the other machine into the Computer Name box and then hit Search.

It should find the other machine if the network is configured correctly.