Windows MessengerWindows Messenger
is an instant messaging
client from Microsoft
that can communicate both via SIP
and .NET Messenger Service
Windows Messenger should not be confused with MSN Messenger (which is limited to .NET Messenger Service), although it looks very similar.
It has a variety of features: instant messaging, presence, file transfer, application sharing, whiteboarding. However, its most notorious feature was the security hole that allowed people to send unwanted adware through it.