|Just to be clear, your pc has nothing to do with sharing the internet.|
This is because you are using a router which you are allowing other to use wirelessly.
By letting other use your router you put them in the same lan segment you are on. This gives them the ability to try and access your system.
If you uncheck file and print sharing [assuming you are the only one using your pc and there are no one you are sharing a printer or your hard drive to] you have effectively cut off network access to your pc.