I use Intervideo WinDVD 5, which I absolutely adore. It comes with some newer computers but if you don't have it, I've seen it on E-Bay and Amazon for rather cheap. When watching a DVD on your computer, it gives you a small camera icon and that you simply press and it gives you a screencap. All you have to do after that is save it with the small save button.

I also know of PowerDVD which I've heard nothing but good things about, so I'm assuming it's an awesome product as well.

Hope that helped :)