On a PC you have to explicitly share a folder for it to be available for sharing. The sharing tab (under folder properties) prompts you for what kind of sharing you want (full control, read, or change), and lets you password protect the share.
Just right-click on the folder you want to share, select properties, and fill in the name you want the folder to show on the network. Click on permissions and select what users (like "everyone") you want to have access, then select what level of permissions you want to allow. Click OK, and you have officially "shared" that folder.
Sharing the root of a drive is "not recommended" for several reasons, and it makes sense to create a folder on the drive you want to share to compartmentalize what you're sharing more neatly.