Can I legally use Microsoft Office icons in designing my site?
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It would appear this is prohibited.
Microsoft product icons are the thumbnail-sized images indicating that a Microsoft product has been installed on your operating system. Icons may not be used in advertising, in books and other printed matter, on clothing or other promotional items, in online and Internet locations, in software applications, in television programs, in commercials, in movies, or on videotape.
You may use Microsoft product icons in training manuals or documentation about a Microsoft product. The use of the icon must be specific to the function of the icon within the Microsoft software. The icon may not be used as a graphical or design element. Icons cannot be modified or altered and must appear as they would within the Microsoft software.
Microsoft makes certain icons available to developers. If you have licensed a Microsoft development tool, review the redistributable section of the EULA to learn which Microsoft properties may be redistributed by licensees. (My emphasis)
...which does raise questions about including the images in the question, although I guess that's fair use.