Taskix is an extremely small Windows program that allows you to reorder the buttons on your taskbar by simply dragging and dropping. If "Group similar taskbar buttons" is set, you can move these groups around and even reorder buttons inside a group. However, you can't move buttons out of a group.
If you are looking for some more functions, then try Taskbar Shuffle. Apart from allowing drag and drop of taskbar items to rearrange them, Taskbar Shuffle also allows you reorder your tray icons in the same way, close buttons with the middle-click and some minor tweaking of taskbar buttons grouping. Taskbar Shuffle takes up a little more memory space compared to Taskix, but I'm sure you can afford 4MB of memory footprint.