Thanks Avi for posting this in Back Page News.
From the Inside Windows Live Messenger Team Blog: Back in November we shipped the final version of Windows Live Messenger 8.5 along with several other Windows Live applications and services. A lot of you have already upgraded, and for that we thank you and hope you're enjoying it as much as we enjoyed building it. Starting soon, we're going to make it easier for those of you who haven't upgraded yet to get onto our latest and greatest.
This will happen in two phases. First, everyone running Windows XP or Windows Vista will get a prompt encouraging you to upgrade. When you accept the upgrade, the new unified installer will launch and start upgrading Messenger. While it's installing, you'll have the option to download and install other Windows Live applications. You don't have to take any of them, but we think you'll like them and hope you'll give them a try.
It will take a couple of weeks for us to get this upgrade out to all users, so if you don't get it right away don't worry, it's coming. If you don't want to wait, you can go to http://get.live.com/messenger/overview to download and install Messenger 8.5 right now.
After everyone's gotten the optional upgrade, those of you still running a beta version of Messenger 8.5 will receive a required upgrade to the final version. When this phase kicks in you'll be blocked from logging in until you complete the upgrade. The final version is even better and more stable than the beta versions, we want you to have the best experience possible! A special note for those of you running Messenger 8.5 Beta on Windows XP 64-bit or Windows 2003 Server: the final version of Messenger 8.5 will not install or run on your OS. We don't want you to get stuck out in the cold so you will not receive the mandatory upgrade to the final version. However, these operating systems won't be supported by future Messenger versions. You may stay on your current beta version, or to get to a more stable final version we recommend that you uninstall Messenger 8.5 and go back to Messenger 8.1.