Again, games that worked yesterday are now "Unreadable"

I know I asked this question already, but everyone stopped answering on it. I tried playing NBA 2K11. Keep in mind that this game is very lightly scratched. I tried playing a game in Association Mode. About 2 minutes into the first quarter, it said "This Disc Is Unreadable." So I went and cleaned the disc. I started it up, tried to play a game, and again it said "The Disc Is Unreadable". So then I tried playing GTA 4. Another message said "This Disc Is Unreadable." So then I tried Battlefield: Bad Company 2, which I just bought 3 days ago, and it couldn't even read it. It said "Play DVD" instead of "Play Game" on the dashboard. I tried talking to an Xpert, but he told me to install the game to my HDD and quickly left. When I tried that with NBA 2K11 it said that it couldnt do so because the game is "Scratched or Dirty." I took off my HDD and it recognized NBA 2K11 but again said the message when I tried to play a quick game. So I took the disc to a friend's house and it worked on his Xbox.