I had the same problem, to get around that after trying everything you have tried, I manually installed the gadgets.

1. Save the new gadget to your directory somewhere easy to find.
2. Extract the file into a new folder called Calendar.Gadget for example, for your gadget will be, NewGadget.Gadget
3. Cut and Paste this folder with all the extracted contents into the Windows Sidebar Directory: C:\Program Files\Windows Sidebar\Gadgets
4. Right click on the Sidebar and Add your new gadget.

Hope this helps, it worked for all my gadgets,
Futureboys