VPN setup help

[tsolve]

Here is my setup:

2 offices, each with public IP address (fixed).
Linksys BEFVP41 as firewall at each office.
VPN tunnel OK between the 2 offices - no problem.
Now I want to be able to connect to either office from home.
At home I have XP pro/Sp2 - behind a Nat cable router.
I have looked at the Linksys documentation, but it assumes that the XP machine is "on the net".
How do I change it to work behind the Nat, and then also I will need the XP machine to initiate the tunnel, not the BEFVP41?

Then, what do I need to do after this - do I need a new "network connection"? - or some sort of VPN client?

Thanks for any advice.
Steve

[chugger93]

If your VPN setup is configured correctly, then all you should need to do is create a new connection under your network connections, select VPN, and type in the server information etc.

[tsolve]

I guess that's where I am stuck. There is no good guide I can find that shows how to do that! I have done what I think I should, but it does not connect - gives me an error 800 - Unable to establish the VP connection ...

When I use Ethereal to watch the traffic I see the following:
A series (3 sets) of TCP msgs that appear to be seeting up pptp, then my XP pc sends out a series of 6 ISAKMP Identity Protection (Main Mode) msgs - no response from the server, and then an ISAKMP Informational msg.

Any clues how to figure this out?

Thanks