Researched it for one of my clients. He was wanting to burn recorded movies and shows to DVD-R on his PC. The DVR encodes the movies/shows in an unreadable format that a computer can't read, for copyright protection.
Unless, someone has made a CODEC for the PC to read them by now. The best place to look would be
AfterDawn.com. I'm not even sure the DVR itself can be accessed over the network. You could try looking in 'Network Places' if you haven't already or download a 'Network Analyzer' run that to find the IP address of the DVR and try and access it manually through an internet browser.
The only possible way I can think of would be to get a video card with inputs for the video and run the sound through your soundcard and watch/record the streaming video that way.
Sorry, I wasn't much help.