That's not the entire story, unfortunately. There are other rules. For example, Windows might be considered a family name, as there are several Windows Operating Systems. Also, this is a technical name, and technical writing often follows different rules, specifically in cases such as this.

For the possessive form of Windows I would look at the Microsoft Manual of Style, which for me is the fourth edition.

Page 184: Possessive Nouns

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Do not use the possessive form of Microsoft. Do not use the possessive form of other company names unless you have no other choice. And do not use the possessive form of a product, service, or feature name. You can use these names as adjectives, or you can use an of construction instead.

Microsoft style:

  the Windows interface

  Microsoft products, services, and technologies

  Word templates

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The correct form of possessive context for words ending in "s," is with a dangling, trailing apostrophe, and omission of the extra "s."

> The correct form of possessive context for words ending in "s," is with a dangling, trailing apostrophe, and omission of the extra "s."

That is correct (or, at least, nearly universally agreed to) for possessives of plural nouns ending in “s”, style guides are mixed when it comes to other nouns ending in “s”, though the most common rule seems to be to use “’s”; while “windows” is plural, “Windows” as the name of the operating system is a proper noun that is not treated as plural.