Each year, one of the most interesting parts of Microsoft's annual Form 10-K filing with the Securities and Exchange Commission is seeing which rivals and products are listed as competitors for each of the company's divisions. The names won't surprise anyone who follows the industry, but their inclusion in the filing means Microsoft considers them formidable enough to warrant a mention to investors.

Microsoft's latest 10-K, filed last week, adds Canonical, distributor of Ubuntu Linux, to the list of acknowledged competitors for Microsoft's Client division, which makes Windows for PCs. Also notable is the addition of Linux distributor Red Hat to the list of Client divison rivals. Previously, Red Hat was mentioned only as a competitor for the Microsoft Business and Server & Tools divisions.

Here's what Microsoft says on the subject in the latest filing:

Client faces strong competition from well-established companies with differing approaches to the PC market. Competing commercial software products, including variants of Unix, are supplied by competitors such as Apple, Canonical, and Red Hat. Apple takes an integrated approach to the PC experience and has made inroads in share, particularly in the U.S. and in the consumer segment. The Linux operating system, which is also derived from Unix and is available without payment under a General Public License, has gained some acceptance, especially in emerging markets, as competitive pressures lead OEMs to reduce costs and new, lower-price PC form-factors gain adoption. Partners such as Hewlett-Packard and Intel have been actively working with alternative Linux-based operating systems.

The Windows operating system also faces competition from alternative platforms and new devices that may reduce consumer demand for traditional PCs. Competitors such as Apple, Google, Mozilla, and Opera Software Company offer software that competes with the Internet Explorer Web browsing capabilities of Windows products. User and usage volumes on mobile devices are increasing around the world relative to the PC. OEMs have been working to make the Google Android mobile operating system more compatible with small form-factor PCs or netbooks.

New in the passage on browser competition are Google, Apple and Opera. Last year's filing listed only Mozilla as a browser rival. Also new in that section is Google Android. The filing doesn't refer to Google Chrome, although Microsoft CEO Steve Ballmer did refer to the search company's upcoming OS in a discussion of Windows rivals at Microsoft's meeting with financial analysts in Redmond last week.

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