Open Windows Explorer, select the menu "Tools | Folder Options", go to the tab "File Types", locate the file type ".doc", then click the button "Advanced", then click "Change Icon..." and input the Word icon file path.

The Word icon file on my computer is "C:\WINNT\Installer\{90110409-6000-11D3-8CFE-0150048383C9}\wordicon.exe"

You should change it accordingly.