A security vulnerability affecting MSN Messenger 6.2, 7.0, and 7.5, as well as Windows Live Messenger 8.0, is the reason Microsoft has forced users to upgrade to Messenger 8.1.
Mary Jo Foley of ZDNet reported that the information was available on Microsoft's security site from September 11th.
A quote from the article at PC World | IDG News Service:
The Messenger vulnerability, which let hackers embed malicious code in Web chat invitations to users, was disclosed in August. It affected MSN Messenger versions 6.2, 7.0, and 7.5, as well as Windows Live Messenger 8.0. Microsoft changed the name of its popular chat client from MSN to Windows Live with version 8.0.
Many users have taken exception to the forced upgrades. Even though it is essential for security, upgrading can have several ramifications when it comes to enterprises and large organizations.
However, the necessity of manually upgrading may be a thing of the past if the ideals of "Webtop" applications are met, which include smooth zero-interruption upgrades.