Thanks for the replies, deanwitty and bigmike7.
From the CD I mentioned, I installed drivers for a pci SATA controller but only to my Win XP install, but I don't think this is the problem--the drive can't read DVDs via the Win XP install on that pc, nor the Win 7 install, nor even boot from DVDs via that drive in the BIOS. I'm not using the drive on that controller (it's PATA/IDE anyway, plugged right into the mobo). Looks more like a hardware issue of some kind. I guess that disc might have scratched the lens, but it was a CD and it and any other CD stills read just fine, it's only DVDs that won't read.
I've got the time, though, so I'll try deleting the drivers anyway, and I'll try reflashing the firmware failing that.
Thanks for the link and info on the LiteOn drive.
So lens cleaners are definite no-no's, but blowing canned air in downward towards the lens is generally considered a viable last-ditch effort?