UPNP service is definitely enabled by default on Pro.

Not on my installation, it's not. I installed XP Pro (slipstreamed SP1a), and the Universal Plug and Play option in Windows XP setup is not checked, and not installed. This is the default for XP Pro AFAICT.

I _do_ have a "Universal Plug and Play Device Host" service, but this is unrelated to detecting UPnP devices on your network, and besides, it's not actually started by default.