Microsoft and IBM have squared off in dueling ad campaigns that underscore the rising threat Linux poses to Microsoft, the No. 1 software maker.

Microsoft recently launched its first paid ad lambasting Linux, the computer operating system Microsoft executives have long derided. Such barbs have done little to slow Linux's swelling popularity.

So Microsoft has begun enticing tech managers to visit a Web site brimming with surveys and testimonials illustrating how its Windows operating systems cost less to own over time than Linux.

"We think it is super important to clarify the facts with that audience," said Martin Taylor, Microsoft's head of platform strategies.

Meanwhile, IBM, long a big Linux backer, is trying to captivate corporate chiefs with a campaign portraying Linux as the future. This month, it will spend 50 cents of every ad dollar on artsy TV spots, print ads and online ads to reinforce that message. While Microsoft woos tech managers reading trade publications, IBM will pitch high-concept messages to executives using venues such as the pro football playoffs and major business publications.

"IBM is saying to CEOs, 'Buy Linux and save your company,' " said Linux consultant Bruce Perens. "Microsoft is saying to technical users, 'Buy Windows and save your job.' "

Practicing what it preaches, IBM says it is studying the feasibility of widely distributing Linux desktop PC programs to its own employees.

It is vital for Microsoft to keep Linux in check. Its future hinges on expanding sales of cutting-edge Windows server software designed to drive richer corporate networks accessed by Windows desktop PCs.

But Linux, little more than a hobbyist pastime a decade ago, increasingly stands in Microsoft's way. Based on open-source code that is continually improved by volunteer programmers worldwide, Linux has emerged the past two years as the fastest-growing server software. It has won converts among government and education users reluctant to rely on a single software supplier. Linux's momentum has spiked:

• In Asia. Database supplier Oracle recently helped broker a joint venture between China's Red Flag Software and Japan's Miracle Linux to co-market a Linux server operating system, dubbed Asianux, for sale in Japan, China, South Korea and Taiwan. Oracle will supply database software to run on Asianux, stiffening competition for Microsoft's server and database offerings in the all-important Asian market.

• On the desktop. Sun Microsystems and Provo-based Novell are ramping up Linux-based desktop PCs and basic programs to offer as an alternative to Microsoft's Windows desktop PC and Office suite. Sun last month rolled out its Linux-based Java Desktop System, and landed government customers in China, England and India. Novell last August acquired Ximian and is hustling to bundle Ximian's Linux-based desktop software into its NetWare server software.

• With cost-conscious customers. The Israeli government recently suspended Microsoft license upgrades, and it is reviewing Linux proposals from IBM and Sun. The city of Austin, Texas, and Massachusetts likewise are reviewing Linux as a cheaper, more flexible alternative to upgrading their Microsoft software.

With more than a 90 percent share of the desktop PC software market and more than $50 billion in cash, Microsoft dwarfs the fragmented Linux movement.

But its anti-Linux ad campaign may come off as "thin and shrill," said Charles King, Sageza Group research director. "Their challenge is to make an argument as to why they are such a good thing, not why Linux is such a bad thing."