Apple has confirmed a glitch that prevents some users from updating their Mac OS X Leopard machines using the built-in software update mechanism.
According to a tech note posted on Apple's Support site, the update process may stop responding while 'Configuring installation' is visible in the update window. The problem happens when software update attempts to install an update that has been partially downloaded.
To get around this problem Apple suggested users first go to /Library/Updates and move the contents of that folder to the trash. You can then use software update to re-download and install the update.
If you don't want to try the automatic update again you can visit Apple's Support site and download the update manually.