I blocked Facebook following the mentioned steps (Add site, Restart Broowser, Clear Cache, Turn off the antivirus). When I tried to remove the Facebook site from the list, there is an error message saying "Unable to update system configuration to reflect the changes. This is probably caused by a security program, such as antivirus or anti-spyware program or you don't have sufficient user level". I can't access Facebook anymore. How can I fix this?