Windows vs OS

Windows and OS both are nouns.

Word	Noun	Adjective	Verb	Adverb
Windows	Yes	No	No	No
OS	Yes	No	No	No

As nouns, OS is a hypernym of Windows; that is, OS is a word with a broader meaning than Windows:

Windows: (trademark) an operating system with a graphical user interface
OS: (computer science) software that controls the execution of computer programs and may provide various services

Other hypernyms of Windows include operating system.

Windows (noun)	OS (noun)
(trademark) an operating system with a graphical user interface	the left eye
	(computer science) software that controls the execution of computer programs and may provide various services