http://www.hauppauge.com/html/usb_data.htm
External tuner? No problem.
Here's the deal tho: Most people have digital cable now, which isn't totally compatible with most PC's TV Tuners. Those tuners are built for analog, not digital, signals. So, you'd have to have the cable companies digital receiver set to the station, pass the output from that through the PC, which can act sorta like a TiVO, and record the signal from the cable box.
I have an older ATI TVWonder Pro, which I do that with, and I cna record MPG from the TV when I want to, but I can't get the digital signal directly from the wall, it has to pass through the cable companie's box first