Like Doc said, imovie and Moviemaker are very much bare bones and would be a good idea to use trial versions of Final Cut and Avid (as a former video editor, I personally recommened Final Cut, even the cheaper version, Final Cut Express, which is what one would call a prosumer version of the pro thing).
One way, I could think for the video clips themselves, is if you record the clips you want from your official DVD to a blank DVD in a DVD recorder. From there you can use the files on the burnt DVD in the computer. I used to do that for actor's showreels. You can then download from the net, for free, a video file converter (my fave is mpeg upstream from Squared). So you may then convert the clips into the type of file required by your editing system (imovie or Final Cut Express, would be easier as mov files are more flexible for compression and decompression, AVI in Moviemaker I have found to be a real hassle and always really hard to compress to fit to discs and Youtube).
Some camcorders, have the function to record directly from VHS or DVD player via AV cables, so check your camcorder's instruction manual and website to make sure, as that would probably be the easiest, and then uploading to your computer as normal.
Remember imovie, Moviemaker and DVDmaker are really consumer software, and there'll always be setbacks with certain aspects such as files (sometimes I suspect it's a way of protecting Hollywood and professionals). I found it enough of a challenge with moviemaker and DVDmaker to do what I wanted it to for the family vacation videos, because of my previous knowledge and work with the professional gear, and it just won't bend as far as it could. In the end I had to compensate and I simply can't afford the good stuff.
