The update "Mac OS X update combined (powerpc)" can't be installed?

Question

I recently upgraded from OS X 10.3.9 to Tiger (10.4.6) and encountered a weird problem. My system is a Dual 1GHz G4 (mirror drive door) with 768 MB RAM.
When I try to launch iTunes, I get an error telling me that "iTunes requires Mac OS X 10.4.7 or later..."
That never happened before, so I assume that the Tiger installation (or a Software Update I was unaware of) sabotaged iTunes by imposing this new requirement.
Now here's the irony: I can't install OS X 10.4.7! When I run Software Update, it tries to install Mac OS X Update Combined (PowerPC) 10.4.10-- but it fails every time with the message:
The update "Mac OS X update combined (powerpc)" can't be installed.
The digital signature for this package is incorrect. The package may have been tampered with or corrupted since being signed by "Apple."
So this seems to be a transmission error, or something wrong with Apple's server. I've tried this repeatedly over the last few days, but nothing changes.
Any suggestions? I tried downloading the installer from Apple's download page, but it results in the same error. Is there perhaps a clean version of 10.4.8 I can download from somewhere?
It's really frustrating not being able to run iTunes because of an OS "upgrade."

Answer 1

Then, stop using Software Update, click the Downloads link at the page's top, and manually download the COMBO update. As for iTunes, apparently you installed a later version. With 10.4.6, your quite a few updates behind: OS, security, iTunes, and QuickTime, just to name a few. Before you update, be sure you make a bootable backup/clone. Details for doing that are in these:
http://docs.info.apple.com/article.html?artnum=106941
http://www.macmaps.com/upgradefaq.html
http://www.macmaps.com/backup.html
http://www.thexlab.com/faqs/installswupdates.html
http://www.thexlab.com/faqs/backuprecovery.html