Microsoft said it would switch off its Windows Live Messenger service on 15 March this year.
On that date Messenger log-ins will no longer work and users must turn to Skype, said Microsoft in an email sent to all Messenger users.
The email also encouraged users to update to Skype and familiarise themselves with the service before the switch-off.
The service switch is a consequence of Microsoft's acquisition of Skype in October 2011 for $8.5 billion.
In November 2012, Microsoft announced that it was switching off Live Messenger in early 2013 but gave no firm date. At the same time, Microsoft made it possible for Messenger users to talk to and swap messages with contacts via Skype.
To help people migrate before 15 March, Microsoft has added an upgrade button to its desktop Messenger that when clicked, uninstalls Messenger and puts Skype in its place.
Until the switch-off date, Messenger would work as it always did, said Microsoft.
The Windows Live Messenger instant messaging program was known as MSN Messenger when it first launched in 1999. The service is believed to be used by about 300 million people every month.