To completely disable the potentially dangerous UPnP service from Windows XP, there are two services present that should be disabled. You can access the Services panel through Administrative Tools (Control Panel>>Performance and Maintenance). The two services you need to disable are:
Universal Plug and Play Device Host
SSDP Discovery Service
Most users don't realize it, but the SSDP service is the actual server that's open to the internet (unless you're firewalled), and the service that turns on the UPnP Device Host. Disable the UPnP Device Host first by double-clicking on the service and selecting "Disabled" in the drop-down menu, instead of the default "Manual." Next, go on to the SSDP Discovery Service, double-click and select to stop the service, then disable it in the same manner.
I hope this helps anyone who might be confused about how to be rid of UPnP.