problem viewing dvd-r

hello, im new here and if this is in the wrong spot i apologize. ive burned alot of dvd movies.. using both dvd-r and dvd+r. Until recently they've both worked fine but now the dvd-r's are not working at all. the dvd+r's are still working and cds. ive tried different media players - wmp11 does not recognize that a disc has been inserted, vlc sees it as a blank audio cd. Power dvd- will bring up an image with audio however it plays very haltingly. someone has suggested to me that this could be a registry problem because i had installed and deleted a program called alcohol 120%. also my OS is windows XP... the pc is less than a year old and the discs worked b4 so i dont think it's a hardware problem. any help would be appreciated. plz reply in as simple terms as possible.. . im very new to computers. thanks.

 

Do the DVD-R discs work in another PC? If they do then it is probably a software problem.