UPDATE: My freeware's latest version also allows you to do this : Windows 7 Start Orb Changer
Windows 7 Start Orb Loader is a free portable tool that allows you to change your Windows 7 start button without editing any system file.
How to use:
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Run the tool as administrator.
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Check Enable Start Orb Loader.
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Click Browse and browse for the folder in which your start orb bitmaps are.
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Select your desired orb from the orb list.
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If you want to access the app directly from your desktop context menu, click Shell integration.
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Click Save Changes.
Now your start button will be changed. Just log off and log in to see the new orb.