There’s a thing that I experienced on two different Macs (Late ’06 MacBook and Late ’08 MacBook Air, both 10.5.7) at the same time: Software Update caused the whole system to hang after I had chosen to install the updates. No app would launch anymore, even a shutdown wasn’t possible.
Deleting /Library/Preferences/com.apple.SoftwareUpdate.plist did the trick and everything’s fine again.