On Windows Vista, I right-click on a folder, Properties, Sharing tab, and either "Advanced Sharing" or "Share"->"Share" causes an error:
Windows File Sharing This program will not run Access is denied. Close
My user account is an "administrator" on this machine.
I think this behavior changed since my computer was added to the new domain recently. I have other folders shared (from long ago), and they continue to be shared just fine.