VPN connection prior to domain login

I have 25 desktop computers that will sent out to separate remote offices and are required to connect to the corporate network through a VPN. I am currently creating an image that will be loaded on to each machine and am trying to find a way to get the VPN connection initiated prior to user login, to ensure that all login scripts, updates, and AV definitions are executed. Due to the number of different locations and the fact that only a single computer will be at each location, static IP?s and router to router tunnels are not really and option. Is there a way to accomplish this with out using expensive client software (Cisco, etc)?

(The servers are running WS2K3, clients are running XP, with a managed firewall appliance)