If having disabled the security feature UAC, then the sidebar and gadgets will stop working (not display).

Windows 7 enforces the sidebar not to run applications that requires administrator rights, but with UAC disabled, then all applications have administrator rights.

To fix this issue, create the following DWORD value:

[HKEY_LOCAL_MACHINE \SOFTWARE \Microsoft \Windows \CurrentVersion \Sidebar \Settings]
AllowElevatedProcess = 1

Credits MyDigitalLife.info