( Courtesy of click here )
It's the messaging service built into Windows which is being used by spammers.(Windows Messenger - not .Net Messenger ).
To get rid of it,turn off the messaging service.
Windows 2000 and XP:- Go Start > Run > type in services.msc.Double-click on Messenger and in the Messenger Properties window, select Stop, then choose Disable as the Startup Type.Click OK.
Windows 95, 98, and ME:- Go to Control Panel and Add/Remove.Select Windows Setup.Select System Tools.Click Details.Uncheck WinPopUp.Click OK.