domain name system - Windows XP clients do not update server 2008 DNS forward lookup zone

I have a Cisco 5505 working as a DHCP server, and a server 2008 DNS server running an AD domain.

I am having problems with all XP computers not updating the forward lookup zone. The reverse lookup zone updates are working. Windows vista and 7 computers update just fine. Additionally the DNS server accepts both secure and non-secure updates.

When people are connected through the Cisco's VPN, they cannot resolve to any machines that have reverse lookup zones, but they can resolve entries in the forward lookup zone.

I have tried ipconfig /registerdns, but the forward lookup zone entries for the XP clients are not being populated.

How can I get the XP Dynamic DNS client to make the updates, or what can I do to debug what's going on?

Thanks