I've just installed a benq dvd rom that works ok as a cd-rom and plays vcd'd but when I try to play a dvd all I get is the audio with no picture. When loading the dvd disc it shows on the player (Win DVD 4) that it is a DVD then changes to PCM and gives out only the audio. I've surfed around trying to find a solution with no luck. Any help will be appreciated

Windows 98SE , 256 Ram , SC Verge PCI (325) CPU 450