Well, as much as the big media companies have tried to influence people to think that when you buy something, you're just buying the physical part, and the digital bits are still theirs, that doesn't make much sense. As Labby pointed out, that should be fair use. You bought it, after all. People rip CDs to their computers all the time, and don't have to pay again to download the mp3s onto their computer (that of course would then be locked DRM and all that, if they're unfortunate).
Would the copyright holders like you to buy it again? Of course! Greed drives a lot of copyright nowadays (in many market systems, such as music, the people who create the product actually get very little money--much of the profit is sucked up by big companies which squander much of it on extras and luxuries for their employees--though independent artists are a different case altogether), and they would love for people to buy things twice so they can get more money for it. Should you have to? No way!
So go ahead, rip 'em and enjoy them. If you're not sharing them, it's simply private use of a product you already purchased. And I can guarantee you that unless you're sharing them online, no one is going to hunt you down and charge you with anything for it. You should be fine.
(If you're curious about all this copyright stuff, there's a really good free--as in public domain--e-book out there by a guy named Lawrence Lessig. He calls it Free Culture, and made it public domain so everyone could freely get it and read it. You can find it easily with a search. It's rather fascinating.)
Enjoy watching!