Are you talking about a DVD cam or a digital camera that records video to memory cards?
If the former you can play your DVD (the video will be .vob files) in your DVD desk top player and hook the A/V out to a camcorder or miniDV tape deck with passthrough capabilites. The cam or deck will make the conversion to AVI which can then be pulled into an editor.
Or, you can get a software program that makes the conversion. There's a bunch of these out there from "free" to a couple hundred dollars. Do a Google search and pick one.
If video is on a memory card you can pull these videos directly into an editing program on your PC that recognizes MPEG2. Once in the editor you can render the video out in any format your editor supports.
MPEG2 is a highly compressed codec and you are going to lose quality of both video and audio when you convert these files to another format regardless whether from a DVD cam or a digital camera on a memory card. If you end up going back to DVD you will lose even more quality through more compression.
DVD cams and video on consumer memory cards are destination formats and not good acquisition formats. They are made to plug your cam or camera into a TV or PC and view, not edit or change. It can be done but it's time consuming and the end results are poor.