Port 1900 is used by the SSDP (Simple Service Discovery Protocol ) Discovery Service in windows XP, ME and other windows operating systems which implement universal plug and play. This service is used to attach to plug and play network devices and services.
Side note: I had set the the ssdp and upnp services to only run when manually started on my client machine running windows XP. I recently installed several updates from Microsoft for Internet Explorer and Msn messenger and shortly noticed alot of connection atempts to port 1900 from my client machine to my linux gateway. Appearantly, one of these updates starts/uses the SSDP service when the windows machine is booted even if the service is set to only be started manually. To prevent the service from running, the startup setting must be set to disabled.
