Can you check something for me please? I'd like to try doing a "Run as administrator" on your iTunes.
Quit iTunes first.
In Computer, open Local Disk C: or whichever drive you have your program files installed on.
If you have a 32-bit Windows, open the "Program files" folder.
If you have a 64-bit Windows, open the "Program Files (x86)" folder.
Open the "iTunes" folder.
Right-click on the iTunes.exe file and select "run as administrator".
Can you enable sharing now?