The encryption on DVDs is called CSS (C ontent S crambling S ystem) and there are a few DVD rippers that can DeCSS (remove the copy protection). This is however against the law in some countries :-).
Anyways - two free programs (probably the best programs also) that can DeCSS:
DVD Decypter - http://www.dvddecrypter.com/
SmartRipper - http://www.afterdawn.com/software/video_software/dvd_rippers/smartripper.cfm