NEW YORK - Linux may sound more like a ''Peanuts'' cartoon character than a serious operating system for business computers. But fast-growing demand for the free software could signal an important new threat to Microsoft Corp., the dominant computer software maker.

In the latest sign of acceptance, International Business Machines Corp. said yesterday it plans to start selling powerful business computers that come preloaded with Linux, a version of the Unix operating system used by companies for tasks such as running networks of smaller computers.

The move was viewed a major endorsement of the fledgling software and comes after similar moves by Compaq Computer Corp., Dell Computer Corp., Hewlett-Packard Co., and Silicon Graphics Inc.

During the past year, Linux has risen to the forefront among the relatively unknown products that can substitute for established Unix programs and, more importantly, for Microsoft's dominant Windows NT software program for corporate computers. Linux made about 17 percent of the software shipped for running business machines last year, up from 7 percent in 1997, says International Data Corp., a high-tech research firm.

However, it remains to be seen whether Linux can break out of its status as a niche product and spur mass demand.

Users rave about Linux's positive attributes, such as its tendency not to crash and its flexibility. Developed in the early 1990s by a Finnish student named Linus Torvalds, Linux can be downloaded for free off the Internet. And unlike Microsoft's software blueprints, Linux's source code is shared over the Web, which makes it easy for programmers to blend in improvements.