Several newer cable STBs allow you to connect and capture video (including stored video, if the box has a PVR built in) via FireWire. However, 5C-encrypted content cannot be captured in this way (it will not negotiate security with a PC client). I don't know of any standalone PVR that supports this (the only reason the cable boxes do is that the FCC says they have to). Content providers don't want you making perfect digital copies of material, *especially* HD material.
Hacking TiVos is a bit beyond the scope of this forum. You might have better luck at an audio-video forum like
www.avsforum.com or
www.hometheaterspot.com, or at a TiVo enthusiast site.