Assuming you’re starting Messenger via a shortcut, the issue you’re seeing with Office when you start Messenger is a feature of the Windows Installer known as Advertised Shortcuts. When one of these shortcuts is used, before starting the application, the Windows Installer will attempt to automatically repair the application and anything else which is it associated with. Microsoft largely gave up on this feature in modern times as what you’re seeing was not that uncommon a problem, but when both Messenger 7 and Office 2007 were made, it was the norm of all the installers of that era.
The quickest fix is to create a new shortcut to Messenger’s msnmsgr.exe yourself and delete the old one. To do so, right-click on the desktop, choose the New option, Shortcut, Select the location of Messenger (C:\Program Files (x86)\MSN Mesesnger\msnmsgr.exe), click Next, click Finish, and then move the shortcut to your start menu, or the taskbar, or wherever you prefer.
Alternatively, if my memory serves me correctly, you could uninstall Office completely and install it again to clear the problem as well.
With regards to creating a topic here via the Messenger tab, you need to apply the following change:
Although I think a full browser window is preferred anyway myself 
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