To add to Bharath's [MSFT] comments here are pages that detail how to setup a PPTP VPN server and client on a Windows XP box. Ignore the comments about IPsec and ports 500 and 50/51 at the end of the server how-to.
http://www.onecomputerguy.com/networking/xp_vpn_server.htm
http://www.onecomputerguy.com/networking/xp_vpn.htm
Also remember you need TCP Port 1723 open on any firewall/router your server PC is behind. The firewall/router also needs to pass GRE Protocol 47 traffic. That is sometimes called PPTP Pass Through. Some consumer grade routers have problems doing that. You can test all this using the PPTP Ping and VPN Traffic sections on this page...
http://technet.microsoft.com/en-us/library/bb877965.aspx
Remember to use a strong password...
http://www.microsoft.com/protect/fraud/passwords/checker.aspx
If the PPTP VPN does not work for you then you might consider Secure Shell [SSH]. SSH is pretty easy to setup on Windows 7 or XP. You can safely access shared files and/or use Remote Desktop or VNC to remotely access/control multiple PCs through the SSH tunnel. In addition you can use a public/private key pair for strong authentication. Here is a small page that talks about this...
http://theillustratednetwork.mvps.org/Ssh/SecureShell.html
MS-MVP Windows Desktop Experience, "When all else fails, read the instructions"