Error Backing Up Dvd's

I am using Nero 6.0 with Windows Xp.
I am trying to back up my Dvd’s and something different happens everytime i write the Dvd.
Most recently i tried to back up a Dvd and Nero got to nearly %60 and failed with error saying something like “Error Reading”
But other time the disc has complete but only has 3/4 of the movie.
Another error was “Able to write disc-at-once”

Any suggestions?
I need help asap as my hard drive is filling up with Dvd’s.

What for a burner and media are you using?