Yeah, you want a vpn connection for the laptop but without knowing all the connection points and configuration of every step it'll be near impossible to tell exactly how to do it.
Most DSL routers come with VPN connectivity options but they're usually infrastructure based VPNs not client server VPNs. This means they're designed to connect one DSL router to another accross the internet to extend the private network behind both DSL routers. A client/server VPN enables the laptop to act as the router at one end and the server at the other, it's a whole other level and uses different technologies. So when a DSL router says it supports VPN connectivity it doesn't usually mean client/server unless it has some sort of certificate authority, RADIUS or similar server of it's own built into it.
If the PC is running XP you want to look for a VPN server application that will run on the XP machine. Maybe some sort of SSL VPN server?