Cool tool: Folder Marker
I have seen all sorts of workarounds in the past too mark which folders are important; renaming folders to start with underscore (_) so they show up first, using numbers or complex hierarchies. FolderMarker is so simple that it is just brilliant; it lets you change the Windows folder icons:
Use predefined icons like "High Priority", "Planned Work" or choose our own icons or colors. A Getting Things Done approach to the file system. Just right click on the folder to change the icon of any folder.