S.T.
Depends on if you have the DVR/tivo type service already with Comcast or if you have an external DVR.
If it's external, go to any Best Buy or Circuit City and ask them for the correct firewire you'll need to connect from the DVR to the box. They should know what kind of a set up you'll need. Remember to tell them exactly what kind of DVR you have.
If it's the Comcast DVR service, you'll need to contact customer service to have a technician walk you through the process as they should have done when you first sign-up for the service.