Okay I understand. Sadly, I cannot find where anything specific has fixed this problem, short of a reinstall of the OS, and we want to avoid that at all costs.
Some folders Windows not allow you to share. Try another folder, like My Documents, and see if you can share that. If not then maybe your server services is not running.
Go to Start > Run
Enter "services.msc"
In the services console scroll down until you find "Server"
Right click select Properties
Change the startup type to "Automatic" and click "Start"
Apply changes and close console
One solution said to help in one case was to uninstall "File and Printer Sharing" and the "Client for Microsoft Networks" then reinstalling them again. These steps may not be word for word for your OS, but hopefully you can follow them through:
Right click on Network Places and select Properties
Right click on Local Area Connection and select Properties
Click to highlight File and Printer Sharing for Microsoft Networks and click Uninstall (do not reboot at this time)
Highlight Client for Microsoft Networks and Click Uninstall (follow prompts to reboot)
After the reboot, repeat the process to reinstall File and Printer Sharing and Client for Microsoft Networks, and follow the prompts to reboot again
Then see if you can share.
Good luck and keep us posted.