If you have a DSL connection with a static WAN IP address, but don't think it's worth the cost, there is another way to allow users to access your server, even if the machines' IPs keep changing. The solution is to use Dynamic DNS. After an account with a Dynamic DNS provider has been set up, you are given a sub-domain name that is associated with a routable IP address. Every time your WAN IP address changes, the provider is notified...
and updates the IP associated with this domain name. The "notifier" in this case is your DSL router, which is connected to your DSL modem.
Now users can remotely connect to your Windows XP Pro machines. All you have to know is your domain name obtained from the DDNS provider.
Many consumer-level DSL routers have this DDNS support built-in. I use DynDNS as my provider because it is free and reliable.