Please refer to this thread:
I'm willing to bet that you may be trying to back up a dvd from Sony, Columbia/TriStar, New Line Cinema, etc... The dvds by Sony/Columbia/TriStar are protected by the new Arccos copyright protection scheme. They are very difficult to copy. It can be done with a little more work. The error message that you are getting is due to this protection. Damn Sony!!!
Your computer is likely fine and has plenty of cpu power and memory to copy the movie. The problem lies with this new protection scheme by companies like Sony and Columbia. Look at the backcover of your dvd. I'll willing to bet that it has Sony or Columbia/TriStar written on it. The print is very fine so look near the bottom of the cover.
I'm also having trouble lately as well. I know for a fact that my computer is not at fault. I maintain it consistently.