If your DVR cable box has an S-Video out and your computer's graphics card has VIVO (Video-In/Video-Out) capabilities, you should be able to with just that an some recording software.
The other option would be to get a Video Capture card for your PC. Use the video out from the DVR to the Video In on the Capture card to record from the DVR.
This *should* work, but never having owned (or rented) a DVR, I don't know for sure. Personally, I've always used my PC with a TV Tuner card as my DVR.
-Wolf sends