media player problems

can anybody help solve my problem,i can play music on realplayer and windows media player no problems but when i come to play videos or dvd's or any mpegs the player whether it be windows or realplayer slows right down and i get a black screen flick on and off and i have to restart my pc, i have unistalled both players and reinstalled them with the newest versions, i have also updated my graphics card with the latest drivers and i have also downloaded different codec packs to see if that works but still no joy, im going nowhere fast can anybody help please
many thanks pap