Xbox Issue: Unable to Read Disc

I have had this Xbox since it first came out and it worked fine. However just recently whenever I put a game into the Xbox it loads very slow. When I get past the Start Menu I usually get an error that says "Unable to Read Disc. Disc may be damaged or Dirty..." If I do manage to get past the start menu i usually get it during the middle of a game so i rarely get to play a full game. Does anyone know what is wrong or how I can fix it? I would appreciate all the help I could get.

Thank You...