Apple has acknowledged an issue encountered by some Leopard users trying to install the company's latest update released Monday.
In a support document posted to its site, Apple described an issue in which a Mac would hang on a "Configuring installation" window when trying to install Mac OS X 10.5.6. That was apparently the result of an incomplete update getting seeded into the Software Update process, and Mac OS X will freak out if it tries to install a partially downloaded update.
Apple recommends that if you encounter this problem, or have already, that you force Software Update to quit, remove the partial update from your library, and re-download the update. If you chose to download the standalone update from Apple's site rather than the combo update, you might also have had this problem; when in doubt, choose the combo update option.
If you've had a separate problem in trying to install the update, check out our sister site MacFixIt for tips and troubleshooting guidelines.