By default, sharing of the root of any drive is disabled (and quite right too, there's almost never any need to and to leave it shared is very bad security).
You CAN however share it if you wish (and at your own risk) - just find the C: drive in Explorer, right click in Properties/Sharing/New share and define a new share. You may receive warnings about sharing a root folder. Ignore them if you must (not a good idea).
Alternatively you can invoke the "Shares Wizard" - Start/Run and put in shrpubw.exe <enter>