Assuming that you are dealing with movies, there are two reasons why this might happen, copy protection or a damaged disk. Newer movies, especcially by sony have bad sectors similar to a PS2 game. They use these bad sectors to screw up DVD copying software like DVD shrink. You can get a utility or rather a kit to copy them anyways, but it is significantly more work than before.
Alternatly your disk could be damaged. In that case, dust off the old DVDXCOPY software and launch DVDXRESCUE. It will rip whatever it can, and in most cases, you will only lose a few seconds.
If its not a movie, Nero has an option to ignore bad sectors and create and ISO anyways. I have used this successfully with PS1 games giving that error.
As for physically fixing your disk, its not easy. The disk doctors usually cause more harm than good. Just make sure there are no smudges and fingerprints.