The remedy proposed by the states would also prohibit Microsoft from using the market power of Windows to strike deals that would limit competitors in other areas, like instant-messaging software.

Although it grew up on desktop PC's, instant-messaging software has gone mobile.

Yahoo and MSN also offer voice calling using their instant-messaging software, but again you need a computer to use them.

Microsoft, which has amassed 1.5 million users of its instant-messaging software in less than a month of service, is not alone.

For users, instant-messaging software typically displays, in a small box on the computer screen, a "buddy list" of friends who are online at any given moment.

Others are rushing to install more secure versions of instant-messaging software that in some cases limit contacts to within the company.

The number of viruses transmitted through instant-messaging software surged in the last year, according to the Internet security firm Postini.

Microsoft also announced early this morning that it was mounting an all-out attack on a market currently dominated by America Online - instant-messaging software.

In New York, Schulte Roth & Zabel, a law firm, has integrated its instant-messaging software with its telephone switching system.

First of all, let's talk about instant-messaging software, or IM.