The program I found to work best is freeware called Auto Gordian Knot
You will also have to download another freeware program called DVD Decrypter
. (It might be that some of the autogk packages have dvddecrypter included in the package. Not sure. Download and install autogk first, then see if you have to download dvddecrypter separately or not.)
Even if the DVD in question is not encrypted, such as your home movies, autogk only deals with movies that have been put on your hard drive in a certain format by dvddecrypter. So you first have to rip the DVD to your hard drive with dvddecrypter, in its IFO mode. (Make sure it's in that mode, as autogk will only accept the directory as input if ripped in IFO mode of dvddecrypter.) Then open the created directory with autogk, you have some choices (target file size, etc.) and options, and it will make an .avi file from that.
When you go to the web page of autogk (URL above), there is a link to its forum, where you can find out more about it. The program's author writes there frequently, and answers users questions.
A good place to start in general about learning about this kind of thing is the website Doom9.org
, and I think also videohelp