Apple has released a Knowledge Base support article that details a fix for an issue that causes Software Update to freeze in the middle of an installation for users running Mac OS X versions 10.5 – 10.5.5.
The problem stems from a bug that allows Apple’s built-in Software Update application to install an update that is only partially downloaded. Obviously no good can come from that.
According to Apple, the solution is fairly simple. Users affected by this issue are recommended to quit (or force quit) Software Update and open up Finder. Choose “Go to Finder” fro the Go menu and type /Library/Updates. Select all of the files in the folder and move them to the Trash. Attempting to open Software Update and download the update again should be successful this time.
A note for people whose computers restarted for their update: If the installation hangs, hold down the power button and restart the system again. Then follow the instructions above. The 10.5.6 update is said to permanently resolve this bug.