You have two options, either a VPN enabled router, or build a fire-wall box with VPN. look into open VPN, it would let you set up one firewall system on LAN2 as the primary, and the other 2 would connect to it as clients. If you set the subnet and address reservations right it will do the job.
OlyComputers
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You don't need to do port forwarding TO a standalone server if it's the first thing on the network. If it's after a router you would, but you can just as easily have it be the first step from the internet connection.
IF you have VPN "servers" at every location you don't need to install anything on client PCs, just the router/server that's bridging.
OlyComputers
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If I remember correctly, there is an XP version but I had some problems setting it up, my system didn't like the virtual network driver :(.
There is a linux based router firmware called DD-WRT, I use it at home. If you have a router that's compatible with it's VPN or "mega" versions it has openVPN built in.
I put it on a $100 Linksys 350N and it works like a charm.
OlyComputers
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