msmsgs runs in the background even when Windows Messenger is not being used.
Solution? Check this out......
http://www.annoyances.org/exec/forum/winxp/n1059731503
BTW, Port 1900 is the Universal Plug and Play port (UPnP). Windows Messenger (and MSN Messenger) use UPnP to dynamically assign ports for various functions in Messenger (voice, Webcan etc.)