Dunno about breaking the law, the law seems very iffy. You're allowed to make a backup for personal use, but how are you to make said copy? Some speculate that making decss was illegal but using it is not, I don't know.
Basically yes you can backup a DVD. You have to rip it to your hard drive first, then if it's over 4.7gig you need to cut the size down some. DVDShrink can do this for free by both stripping out unneeded parts (how many audio tracks do you REALLY need?) and shrinking the movie itself. There are other programs out there that may get you better quality results, but DVDShrink is free.