In windows explorer right-click on your C: drive and choose properties.

Go to Sharing tab and choose advanced sharing, then Permissions.
I believe what you need is group Everyone. Sharing from the root drive is discouraged because it allows for total access so to me it looks like it is treated diferently than if I share one of my own folders. As I said I just got into this last week when I added a wireless router to link to the Home Theater PC (HTPC) I added in the living room so I'm learning here too.
Note: Windows 7 does not allow the sharing name to include a $ after the letter like in older versions, therefore you cannot share this as C$.