Over the next few weeks there will be an upgrade to the Windows Live messenger service and the upgrade will happen in two phases as detailed below.

The upgrade will include fixes as well as resolve a security vulnerabilities as detailed in Security Advisory 973882

Below is the timeline of the upgrade from  the Windows Live Messenger blog

First Phase, Optional Upgrade:
The optional upgrade will happen in two stages:
Starting Aug. 25, customers using versions 8.1 or 8.5 were asked to upgrade their client.
Starting early Oct., all customers using versions 14.0 (but not the latest release 14.0.8089) will be asked to upgrade their client.
The upgrade at this time is optional. Customers who haven’t upgraded during the optional phase will be required to do so during the second phase.

Second Phase, Mandatory Upgrade:
The mandatory upgrade will happen in three stages:
Starting mid-Sept., all customers using Messenger 8.1 or 8.5 will be required to upgrade their version of Windows Live Messenger.
Starting late Oct., all customers using Messenger 14.0 will be required to upgrade their version of Windows Live Messenger.
To ensure that we are protecting customers, those who do not administer the upgrade will not be able to sign in to Messenger after this time.