Why won't some DVDs play in my computer's DVD drive?

I have a small collection of commercial movie DVDs. Most of them are from the U.S. market place and some are from Asia... *_ALL_* of these DVDs play in my Phillips DVD player that is connected to my TV. But only *_ some_* of these DVDs will play in my DVD player that is in my Dell Inspiron 531 computer with Windows 7 Ultimate. This computer has a *SAMSUNG HD161HJ SCSI DVD/CD Disk...