1) sharing works on mutliple system attached to the SAME router, not across the Internet.
2) let all systems find their IP addresses using DHCP
3) sharing depends upon the firewall exceptions (all systems) to allow Print/File Sharing
4) needs all systems {Xp, Vista, Win7} to use the same workgroup name and sadly, you will need to change it in two of the three listed
5) you need to enable Simple File Sharing (all systems)
6) you can use the Guest account for access to Shared Documents folder, but need
the user/pwd of the target system to access other folders via Map Network Drives