Problem pinging computer on a domain using Vista Business

I have 2 laptops set up at home on the same workgroup using the same
router.

Laptop1 - XP Pro SP2
Laptop2 - Vista Business

Both machines are running the same version of Norton 360 Antivirus.
Both machines have Windows Firewall turned off.

With both machines I VPN into the client gatway successfully. I try
to ping a computer on a network, let's call it: [MachineXP.domain]

Laptop1 can successfully ping [MachineXP.domain].
Laptop2 running Vista Business can not.

Message returned from ping (Ping request could not find host
[MachineXP.domain]. Please check the name and try again.

One more interesting note: I can ping the internal IP of that
machine. So the response from the ping on Laptop1 gives me the
internal IP Address of [MachineXP.domain]. I can then ping that IP
address from Laptop2.

Any thoughts???