Apple has posted a support article explaining how to recover (by deleting partially downloaded files) if Software Update stops responding during its "Configuring installation" phase. The bug is fixed in Mac OS X 10.5.6, but can still affect the 10.5.6 update process. follow link
Keyboard Shortcuts in Stacks
You aren't limited to using the mouse or trackpad to select items within a stack. Once you've clicked a stack to open it, you can type a letter to jump to the first file starting with that letter, or press either the Tab or arrow keys to move among the stack's contents. Press Return to open the selected item.
Visit plucky tree
Workaround for Software Update Hang While Installing