This text is from https://www.microsoft.com/en-us/legal/intellectualproperty/permissions
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 video.
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. (Find more information about how to buy Microsoft developer products.) 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.
Now obviously I can't take the icons from Word, Excel, etc. and use them in my own applications, but am I allowed to use them in an application to show what the finished product could look like?
For context, I'm developing internal in-house applications (front end for some databases, reporting software, etc.) for a company and I'm considering using the Microsoft icons as placeholders icons when I show prototypes of the software to my superior since they are similar to the icons I've considered purchasing for use in the application.
Is there anything legally that prevents me from doing this?
For clarification, the icons will not be used in the production version of the application. I only want to use them to show what the finished product will look like. (or similar to)