I know this thread is a year old but regardless it might help if I put in my 2¢.
First: If you have basic cable getting a digital capture card and running the coax directly into your computer will give you everything the cable box will unencrypted required by law in the US. However, if you do not want to get a tuner card plugging a firewire cable from your cable box to the computer will allow you to capture the same channels. Personally, I find this silly. Why rent a cable box for $X a month when none is required?
Second: If you have extended cable or better (premium) aka a service that requires you to get a cable box to watch all of the channels you're paying for then there are currently two ways to get those channels on to your computer in regards to LIVE TV.
1) A cable card tuner. ATM ATI is the only company on the market that has a cable card tuner for sale outside of OEM. CAUTION: You will have to have a specialized motherboard if you go this route. It probably would be easier to buy a pre made Dell (or other brand) with the cable card option available than making your own white box. Also, this will only work in Windows Media Center and your content will be encrypted. If you're curious here is a list of computers with cable card compatibility:
http://ati.amd.com/products/tvwonderdigital/partners.html
2) An HD-PVR + IRblaster. Currently the only one on the market is:
http://www.hauppauge.com/site/products/data_hdpvr.html Basically, it will capture full HD (up to 1080i) and output it to USB in the h264 format. This is not a digital 1 to 1 capture, and you still have to have a cable box from your cable company but if you're paying for HBO HD you can fully capture that channel. Heck, if you pay for ppv porn you could even capture it with this. You can't do that with the cable card method.
There are a couple of other solutions like buying a TiVO and streaming your captures to your computer over the network, but I believe this does not support live TV. (I could be wrong.)
So, of the two ways to do this the pricing is:
CableCARD: $200 for each input (aka watch one show, record another would be $400) + $2.05 a month for each cable card + remote + cost for computer HW. This is extremely restrictive to specific computer hardware ($$) and only works on wmc.
HD-PVR: $250 for each input + a monthly cost of usually $6 for each cable box (or a one time payment to buy a cable box with $2 a month for the cable card) + IRblaster + any computer hardware that can handle USB. bonus: less restrictive.
And the third option: Buy a dedicated DVR like a TiVO. Usually every fathers day they have a 1 day sale and a dual channel HD TiVO with lifetime subscription is usually around $500.
Hopefully this helps.