OK, do you have the installation discs that came with that iBook? Boot from those and run the Disk Utility and repair the disk and permissions.
If that doesn't work, then you might have to do a reinstallation. THe good thing is that you can do an Archive and Install which preserves your documents and data but only replaces the System folder. This means that you owuld have to perform the updates once again. Might I recommend downloading the Combo Updater for the latest version of OS X that you have? For example, if you're running 10.3, download the 10.3.9 Combo Updater manually from the Apple website. If it's 10.4, then download the 10.4.9 Combo Updater (make sure it's the PowerPC version!). I've had better results with a manually downloaded updater than updating through Software Update.
Also, make sure that whenever you perform updates that the only thing connected to your Mac is your keyboard and mouse. Since this is a laptop, just leave everything else unplugged when performing updates.
Hope this helps.