You may not be able to directly 'copy' a dvr recording into your computer, but you should be able to record your dvr's analogue output to your computer. Basically, you would playback a dvr recording and capture it to your computer in real time, as standard definition video.
You'll need a video card capable of video capture, or a stand alone box like the Canopus ADVC models (which are superb), and some software for capturing the video coming into your PC. My favorite combo for this is a Canopus unit and the Scenalyzer software. Be aware that you may need a lot of hard disk space if you are capturing the video in raw, uncompressed DV/AVI format. After capturing, you can then edit, compress, and even burn a dvd of the video with a video editor, like Adobe Premier or Sony Vegas.
Once you get the hang of it, it is pretty easy, but can be time consuming.
Hepcat
