The icons that are displayed on the folders are set through the hidden desktop.ini file inside of each folder.
Fix for Special Folders Reverting to Default Folder Icon in Windows Vista
My inbox has been flooded with people asking why the "pretty" icons in their user folder keep turning back into regular folder icons, and what they can do to fix it. After writing the first article about the Music folder, I decided to just put all of the information into a single article.
