REDMOND, Wash. – Microsoft Corporation’s latest instant messaging program is ready for prime time, the company said yesterday. Windows Live Messenger, which had been available in test form, will officially launch today.

The free program is an upgrade to MSN Messenger, the previous name for the software maker’s tool for quickly communicating online. Such products have become incredibly popular for a wide variety of audiences, from teens to business people. The basic service gives people a way to quickly type messages back and forth. Windows Live Messenger also offers video calling and ways to easily share files, among other features.

Competitors, including market leader AOL, also are adding such functions. Yesterday, Yahoo Incorporated said it was opening up its messaging service so that outside developers can create programs that plug into it.

Windows Live Messenger is one of the first products to debut under Microsoft’s new “Live” brand. The company is renaming some products and launching others under the “Live” brand as part of an overall push to offer more Web-based products. It faces intense competition in the field from companies including Google and Yahoo.