The latest versions of the Messenger application programming interfaces (APIs) documentation. Microsoft has developed three sets of APIs that can be used to create custom Messenger applications. For example, you can add images of your contacts or create an add-in component that links to your e-mail system to alert you when new messages arrive.

Messenger Version - 4.6 for Windows 95 & 98, NT 4.0 & 2000, Windows Me, Win XP
Release Date - 12 Dec 2001

Messenger.exe - 249 Kb
MessengerAddIn.exe - 121 Kb
MessengerUA.exe - 295 Kb