This document describes how to get Openswan working with various other IPSec stacks, including Openswan and Windows 2000/XP. If you have any difficulties with this process, please e-mail the Openswan mailing list, or if you can’t get help from there, e-mail me at: firstname.lastname@example.org. If you are using clients which benefit from receiving an IP address on the remote network (Windows, PocketPC, etc), you may want to consider L2TP over IPSec instead of the method described below. Jacco de Leeuw’s pages cover this in a good amount of detail; I also have a basic walkthrough available at my L2TP-over-IPsec page.
If you’re not sure if IPSec is right for you, I have written a quick document about some of the various types of VPN available under Linux. It is available at: http://www.natecarlson.com/linux/linux-vpn.php. I hope this helps clear up some questions.