You could buy a used ReplayTv with lifetime subscription on eBay for about $150. Your HD box would need to have composite or S-Video out in order to send the signal to the ReplayTv. If the ReplayTv has the proper code for the Comcast box, it will change the channel on the Comcast box via an RF emitter. If not, you will have to have the HD box set to the proper channel in order to record the proper program on the ReplayTV. The RTV will have the proper program description, but the Comcast box must be on the proper channel.
The same applies to an older Tivo Series 1 or 2. The later versions of Series 2 have two tuners, allowing you to record two shows and watch another one already recorded.