According to the Windows Live Messenger blog, users of Windows XP and Vista will soon (if they haven't already), be forced to upgrade to WLMessenger 8.1 for security reasons described in security bulletin MS07-054. "Some of you might feel this inconvenient, but in order to protect you and protect the health of the network, we have chosen to take this step," the blog entry said. If a user prefers to wait for the notice instead of installing the newer client manually, Microsoft recommends avoiding accepting a webcam or a video call invitation from an untrustworthy contact.
That means users can no longer use MSN Messenger 6.2, 7.0 and 7.5 or WLMessenger 8.0 as these versions are all affected by the security vulnerability. Windows Live Messenger 8.1 and 8.5 beta are not. Because it is a security issue, Microsoft has decided that to protect its users by making the update mandatory. Since January 2007, when trying to log in to the chat client, the notice to upgrade has been optional. Since the Messenger network uses a rolling upgrade mechanism across the network, it will take several days for all users to receive the mandatory upgrade notice. Users that are running Windows 2000 will need to use an updated version of MSN Messenger 7.0 because Windows 2000 isn't supported by version 8.1. Windows 2000 users will be upgraded after the Windows Live Messenger upgrades.
News source: Inside Windows Live Messenger