Local Security Settings – Access this..

Local Security Settings – Access this computer from the network

I have a problem getting the Local Security Settings – Access this computer from the network to persist. As a result, the computer’s shares are only available for a short while – the setting eventually gets ‘blanked out’. I log into this machine as an Administrator, set the policy to Everyone, Administrators and Users, and the setting stays – for awhile…then gets reset to nothing ???? The machine is A Win XP pro machine. Anyone encountered this problem before?