Apple Software Update error- "Entry Point not found"

I have had Apple Software Update/ iTunes on my PC for over 4 years. Today, randomly, when I went to update iTunes to the new 10.5 version, Software Update did not prompt me to update. That was odd, but I didn't realize anything was amiss yet at this point.

In iTunes, I went Help > Check for Updates which indicated the new version was downloaded. When promted whether I'd like to update, I hit yes, which even more oddly took me to apple.com to download iTunes all over again. (Normally, updates download from within iTunes and it's an update, not a full hour-long re-download.)

I didn't want to redownload iTunes (what would happen to all of my previously downloaded apps and music, set playlists?) so I opened Software Update, and got the following error message:

SoftwareUpdate.exe - Entry Point Not Found

The procedure entry point ?CreateProductsFromXML@CSUCatalogObject@@QAEXP6AIPAXIIJ@ZOIV?@CStringT@WV?$StrT raitMFC@WV?ChTraitsCRT@W@ATL@@@@@ATL@@@Z could not be located in the dynamic link library SoftwareUpdateFiles.dll

Strange, right?

So my questions.

1.) Does anyone know why Apple Software Update would suddenly get errors? I've used it recently (this past weekend?) and everything was fine.

2.) Is there a way to fix Software Update, since I'll surely need it in the future.

3.) In the meantime, should I proceed with re-downloading the full version of iTunes again, and will it mess up anything I currently have in my iTunes library? Would I need to back it up or do anything to help ensure there are no further problems?

Thanks for your help. To say I'm confused is an understatement!