PowerBook continuously reboots after running Software Update

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This morning I ran Software Update on my Powerbook. There were about 5-6 items including the latest Mac OS X update, Java, iTunes and a couple other things. The update scripts ran without incident. However, when it tries to restart, it keeps rebooting itself over and over. I first get a black screen, then the gray screen with the Apple logo in the middle, then the spinning circle comes up and the machine reboots again. There were no error messages. I have to hold down the power button to shut down the machine and break the rebooting cycle. Any ideas of what I could try?
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Replay

Can you boot from the OSX install DVD? Then Disk Utility may be able to repair it.
Can you boot in Safe mode, by holding down the Shift key?
http://support.apple.com/kb/HT1455?viewlocale=en_US
You may be able to start and use Disk Utility for repairs.
Sometimes, downloading the Combo Updater of any OSX update and installing it works better than Software Update, for no good reason.