These are original movie DVDs, and the ones that it struggled with are copy protected. As everyone knows, DRM is starting to become commonplace.
My resolution was to download this file:
And run "cdgone.reg" from it.
The "EditReg.reg" file changes Windows so when you double-click a *.reg file, it opens in Notepad rather than automatically merging (kinda scary "feature", there, Microsoft).
I also renamed c:\windows\system32\drivers\PXHelp20.sys to PXHelp20.sys.old since it seems that many sites referencing this file feel it's buggy.
Wa-Bam!, I can read movie DVDs again - no reboot even required!!
The copy-protected DVDs take a while to load (the drive grinds on 'em), but then they come up.
I had tried several other solutions, include removing UpperFilter and LowerFilter from this registry entry, but it didn't work.
I also tried Apple's solution of removing LowFilter and using their GEARs.... driver in the HighFilter, no joy.
I may have to reinstall some of my software if it won't read the drives any more, but the couple I've tried seem unaffected so far. It's early yet.
cdgone.reg (for reference in case it stops being hosted)