Anyone had any luck setting one up?

Here's what my setup looks like: DSL Modem --> PIX 501 Firewall --> Switch --> Server (Windows Server 2003) --> Client Systems (XP Pro).

The modem has a default IP address of 192.168.0.1. My network is at 192.168.1.#. When I change the IP address to 192.168.1.8 to match the network, it loses Ethernet connectivity-- it doesn't show up in the DHCP addresses leases, none of the systems are able to access the Internet, and I cannot interface with it over the LAN. (I have to connect a notebook PC directly to the modem and change the IP back to default to restore connectivity.)

I don't even know if I'm looking in the right place. I just know that I need to get VPN available for workers at our other locations (we're a non-profit in Eastern New Mexico).

Any and all help will be greatly appreciated. Thank you.