If you have ever read a list of funny directions and warnings then you will get the joke I found on Apple's support page.
While trying to upgrade a new power book the system suddenly stopped responding and I got the spinning rainbow of death.
I force quit the update and restarted, thinking to try again. Well after about 6 times I started looking for answers, what I found was a logic loop on the apple support page saying:
"To avoid this issue, download and install the Mac OS X 10.5.6 Update or later. Note that this update must first be installed to avoid the issue; the issue may occur while downloading the Mac OS X 10.5.6 Update or later."
Read the whole tech bulletin here.
When attempting to update software using Software Update in Mac OS X 10.5, the update process may stop responding while "Configuring installation" is visible in the update window.
This can occur if Software Update attempts to install an update that was only partially downloaded.