Problems dialling in to a home VPN

I've recently installed a Draytek 2820n router and am trying to set up a
VPN connection. In fact I *have* set one up but there is one little
problem, a firewall problem.

My home network consists of a couple of printers, an NAS and three
computers running XP. I have two roving computers, one is a laptop
running Vista and the other is a netbook running XP. The Vista laptop
can dial in (using 3G) to the VPN with no problem at all. It all works.
I can access all the computers on the LAN

The problem is with the netbook running XP. I can get the Draytek to
assign an IP address on the local network. I can ping all the computers.
I can access web-based pages on the printers and the NAS. What I cannot
do is to access the XP computers on the LAN *unless* I take the firewall
down on the netbook. I've tried to discover if it's a port on the
netbook XP blocking communication without success. I set up the VPN
connection using the XP wizard which should have taken care of
everything.

The mysterious thing is that the Vista laptop works *with* the firewall
active. The XP netbook works only with the firewall de-activated. I've
googled for help but no-one else seems to have had this problem. Does
anyone here know what I should look for?

Thanks in advance, David