This article can help you Configuring PHP with MySQL for Apache 2 or IIS in Windows. Look at the section "Configure PHP and MySQL under Apache 2", point 3:
extension_dir = "c:\php\extensions" ; FOR PHP 4 ONLY
extension_dir = "c:\php\ext" ; FOR PHP 5 ONLY
You must uncomment extension_dir param line and set it to absolute path to the PHP extensions directory.