SOME DVDS not detected by dvd drive

[aesop64]

I have noticed another post simular but not the same. Hope this is O.K. !
I have a Dell Dimension 4600, Running XP and I recently installed Service Pack 2. My DVD Drive is a HL-DT-ST DVD-ROM GDR8162B
I have never noticed any problems with any DVDs before, Now some dvds are not detected by my drive they spin & spin but never play(like trying to play a blank cd-r).Others are detected and play excellent. When one of the undetected dvds is in the drive, clicking the dvd player icon in My Computer or manually open (or play) and any thing else I try all say the DVD drive is empty or insert a disc! The DVDs are not -/+Rs or -/+RWs just regular movies. I have tried opening the DVD decoder or player software first and then inserting the disc, Same result with all of the following Dell's PowerDVD ,Dell's media experience, Microsofts Media player, Interactual, and a few other programs I probably should not mention. As I said earlier the ONLY new software or hardware is Service Pack 2 which I reluctantly installed. I tried the trouble shooting guide on my PC but of course nothing really applied I also contacted DisneyDVD technical support because one of thier DVDs was one that would not play. They were very accommodating but nothing of value was gained in our conversation. The other DVDs that would not play were HBO's Curb Your Enthusiasim Volume 1 & 2. Gangs of New York, Hocus Pocus ect. ect. All of the DVDs will play on my stand-alone DVD player (for TV). Of course I want to get to the bottom of this! Any help would be greatly appreciated!!!