If the word "windows" is used in conjunction with another word/s to make a domain name that is internet/computer related, such as windowscomputer or windowsbrowser etc, then yes, it would be probably be trademarked by Microsoft.
Of course, you can use "windows" in domain names for other things like newwindows, doubleglazedwindows, sashwindows etc, none of which are trademark infringing against Windows.