Basic networking between two locations

A small company has 2 offices, with only 2 PC’s at each office. They want to be able to share data between the two offices. All PCs are running Windows XP and both locations have cable internet. What would be the cheapest and least complicated way to do this? I was thinking about using the built in VPN feature in Windows XP, but have never used it before and was wondering if that would be the best way to go, or if a VPN router and firewall should be purchased.