Play DVD on PC

I have Dell Dimension 4600 with dual drives. I can’t get anything to play on dvd drive (16X DVD-ROM ). Troubleshooter said I needed a dvd decoder. Downloaded Sonic Cineplayer DVD Decoder Pack 2.3.1 . Still NOTHING ? I can’t get any answer from Mfgr…..Need help… I’m running Windows XP (home). Thanks, Ed .