The best option is to uninstall them in the gadgets menu. A right-click on the desktop and the selection of Gadgets opens that menu and un-install it from there.
It can happen that a gadget cannot be uninstalled properly from the gadgets interface. Deleting the file in the gadgets directory is then an option to remove it from the operating system.
These are the two locations
System Wide:
C:\Program Files\Windows Sidebar\Gadgets or for 64-bit systems in the folders C:\Program Files (x86)\Windows Sidebar\Shared Gadgets and C:\Program Files (x86)\Windows Sidebar\Gadgets
User Specific:
C:\Users\username\AppData\Local\Microsoft\Windows Sidebar\Gadgets
Regards
David
reference: http://www.ghacks.net/2011/01/04/how-to-uninstall-gadgets-in-windows-7/