VPN using Cisco 803 ruta & Win 2000 Prof

I have managed to connect My HQ to ISP (has w2k server)using Cisco 803 ruta on Basic ISDN. I have 5 remote areas with 5 users each. I have procured the same ruta and Basic ISDN for them as well. I would like to connect through the same ISP and create a VPN link using the internet as the Backbone. How do I do the configs.

Please help