Dvr that will work with comcast

You can still record on your VCR using the old style RF or "channel 3" output of the cable box. Connect this to the antenna in of the VCR and set the VCR on channel 3 and you can record the channel that the cable box is playing. What you cannot do is watch and record different channels or time shift unless you remember to leave the cable box on the correct channel. I don't think there is an inexpensive solution that will give you full functionality. You may be able to find a DVD/Hard Drive recorder that can be programmed to change the channels on the cable box according to your recording schedule but that would cost you about $400. It would not record in High Def.

The Tivo Series 3 or higher will take a CableCard, but you're still paying $2 for the card and $13 for Tivo.
There are a lot of old Sony SXRD demo boxes floating around from Best Buy and Circuit City displays (Sony DHG-HDD250) which are actually stand-alone DVRs - they take a CableCard and use free TV Guide service. If you look hard enough on eBay or Craigslist you will find one.