You need a tv tuner card....like from hauppauge. Most, if not all, cards can handle a cable signal and decode it easily for muliple tuning. I'm not sure how a tv tuner decodes cable, but it should work. Im not sure about digital cable or HDTV cable because your in satellite dish territory without the satellite dish.
You are also trying to build a unit at the age of deliberation where a democraticy is being overthrown by capitalism and where you no longer own anything......you are purchase a liscense to use it.
Now.....how do you liscense a toilet...only persons of that household can use it, if any guests want to use your toilet then you must purchase a public liscense or get a buggy open source toilet.
Cable\satellite providers and hollywood studios are working with microsoft on DRM and HDCP. DirecTV and Dish are developing satellite tuner cards. Also, there are numerous companies that are developing HDCP\HDMI capture cards. So you will be able to run encrypted video to a computer to record it only on a DRM format which cannot be re-encoded, tampered, burn on a dvd, or shared online, etc....
Yet again, regular cable should not be a problem.
You could do HD with a cable box with serial port, RGB or component capture card, and some software written to control that cable box.
Also, rental liscenses do not allow you to make a copy for yourself.
This is a warez topic an as warez topics they are illegal and are probably prohibited and moderated.
As for DVDs you own,
you can use Media Portal on XP.