I want to record TV program on my PC, the set-top box has CVBS and audio (RCA), HDMI, YPbPr, RS232, USB(a female one) and Ethernet port, and my PC has VGA, USB, 1394 port and PCI Express slot. The set-top box Wikipedia article said it could act as a tuner, how can I connect them? Thanks.
I don't know what set-top box you have, but I've not heard of one that works without a computer that can connect directly to a computer without additional hardware that most computers do not have.
I also don't know what service you use to get television signal (Satellite, Cable, FiOS, or is it "over-the-air").
And I don't know where you are as digital rights technologies mean many places won't work with certain technologies.
I'm in the USA and use a Ceton CableCard Tuner card. My Cable Television wire connects directly to the Ceton card installed in the computer and a CableCard from my Cable Television provider allows the Ceton card to receive all my channels in HD. This allows Windows Media Center to record up to four different programs at once like any DVR might. However, as I understand it, CableCard (and the Ceton card) only works with Cable Television providers (and FiOS) in the United States. It does NOT work with Satellite and it does not work outside the United States, including Canada.
There are other Tuner products and they are (in my opinion) generally inexpensive. Hauppauge makes several different ones as does ATI. The devices tend to cost between $50 and $200 depending on the number of tuners and technologies supported.
If you can be more specific as to what you have and where you are, better advice may follow.
Bottom line - I don't know of any way to do what you're asking without at least one more piece of hardware and where you and what you have will determine what that hardware can/needs to be.
The 1394 port probably supports video out. (IIRC, It's required to by law.) http://www.mythtv.org/wiki/Firewire_Cable_Box_Compatibility
However, the premium channels are likely encrypted.