Prevent thumbnails from rearranging on the Windows 7 taskbar?
Windows 7 by default groups taskbar buttons. Even if you specify in the Taskbar properties to "Never combine," the same process icons will always be grouped and moved as a group.
The "7+ Taskbar Tweaker" program allows you to not group same-process icons. With those settings, it is possible to manually rearrange your icons no matter what.
edit: I re-read your question and see what you're asking more clearly. The same program has the option to "Drag within groups with right-click." This function should give you the ability to rearrange the window thumbnails.