You may need to close the lock icon on the policy or clear out the corrupt defs, either of this should fix the issue
1) Open and Login to the Symantec Endpoint Protection Manager
2) Go to 'Clients' and Select the Right group to which the Client belong to / report to.
3) Click on 'Policies' Tab
4) Make sure the check box that says 'Inherit policies and Settings from parent group' is unchecked.
5) Double Click on Antivirus & Antispyware policy [Unshared]
6) Click on Edit shared Button
7) Uncheck on 'File System Auto Protect' or/and Unlock the Lock next to it.
8) Wait for few Seconds and then check the client.
Clear corrupt defs and symantec
http://www.symantec.com/business/support/index?page=content&id=HOWTO59193
Event ID 42: Auto-Protect Error: Auto-Protect is disabled because registration of the virus databases failed
http://www.symantec.com/business/support/index?page=content&id=TECH139729