To enable APC extension on Ubuntu, Use the following command
sudo apt-get install php-apc
To enalbe APC extension on Windows,
1) Go to http://downloads.php.net/pierre/
2) Download php_apc-
3) Copy php_apc.dll to /ext folder
4) In your php.ini, make sure you have: extension=php_apc.dll
5) Restart your server.