It depends, which I'm sure is just the answer you were looking for.
Basically there's a big difference between OTA broadcasts and cable. A tuner card should be able to get OTA broadcasts without any issue, but cable companies tend to encode signals in such a way that you need their special hardware box to be able to get a watchable signal.
There may be a few channels that are "in the clear" and you can use without issue, but probably the bulk of channels will not work. To get some idea what will and will not work, just connect the cable feed directly to your TV and have it scan for channels. Out of probably a few hundred channels, you'll probably only find a dozen or so that you can actually watch without the cable box. If you're lucky, those will be the channels you care about, but otherwise you should plan on having to pass everything through the cable box.
well that's the problem, i connect just my cable to tv and it has no channels without cable box.
damn it so if i pass everything through the cable box i will still be able to watch it on tv and pc? at the same time or no?
You can get an HD tuner card that can get over the air (OTA) HD broadcasts which are from your local stations. To get scrambled cable HD channels you need to route them through your cable box which means you need to have it on the same channel. But you can get the video on the computer though that card.
Ceton makes a PCI card called the InfiniTV (Link) that allows you to get HD cable channels without the need for the cable box. However, you would still need to rent a "CableCard" from your cable provider.
I've never used it so I can't recommend the card but I was trying to do a similar thing as you last year and this was the solution I came up with. I ultimately decided it wasn't worth the price/hassle and just got a DVR through my cable provider (Cablevision) for about $10 month.