Nec 3520a - Not Reading Game Dvd

All of my games programmes have stopped recognising my DVD player when I try to run the game. I can install games from the DVD and see what is on each disk using explorer, but everytime I try and run the game I get a message saying I need to put the orginal DVD in the drive or that it can’t find the DVD. My DVD is an NEC ND 3520A. I am running a fully up to date version of XP Home. I can still write CD’s and DVD’s using the drive. My gut feeling is that this is a software problem. Any help would be appreciated.