There are basically 3 different types of PC TV card ;
- analogue
- digital terrestrial (Freeview)
- digital satellite
If you just want to watch Freeview you need a digital card and you simply connect a regular areial lead. I am not aware of any PC products that support cable TV.
Problem is, if you wanted to use the output from the cable box it would need to connect to an analogue TV card in the PC. That is assuming your cable box has an aerial IN and OUT.
But then I guess you get all the Freeview channels on cable anyway so no need for a digital PC card.
You need fairly decent CPU, video card, sound and memory. Also a good signal level as TV cards are not as sensitive as TVs and digiboxes.
There are also external USB devices available.