Have you ever somehow managed to break the Gadget file association in Windows Vista? I have somehow, and that means that you can no longer install gadgets. Double-clicking a .gadget file does nothing, and the Sidebar offers no option to install gadgets itself!
If you try to re-associate the files with Windows Sidebar by telling to Open With… and navigating to the Sidebar executable, you’ll have found that that doesn’t help — all it does is bring the Sidebar to focus, but doesn’t install the gadget.
Fortunately, help is at hand: all you need to do is download the following archive *and apply the registry entries by *double-clicking the .reg file inside the archive, and saying Yes at the prompt.
All I did was export the corresponding keys from the registry on a Vista machine with a working Sidebar — nothing else!
Hope this helps others like it has me.