Vous pouvez do this with 7 Taskbar Tweaker.
Just vérifiez le box Drag to reorder under Thumbnails section:
As stated on the app, it's allowing you to reorder from the thumbnail: hover the app on la barre des tâches and wait until the windows' thumbnail is shown, then click-and-drag the thumbnail to reorder it. The corresponding window on la barre des tâches will follow the order of the thumbnail.
The previously mentioned Taskbar Shuffle doesn't support Windows 7 yet, to my knowledge.
