Since this is on an Exchange Server, you may want to check with your admin to see if they disabled the feature for some reason.

Found this in another post:

http://support.microsoft.com/kb/q258277/, is a link to microsoft saying to add the entry to disable pst sharing, so a natural assumption would be to delete it, to restore it. However, if it is implemented as part of domain policy then it will just reset with the next boot.