First of all you need an RF or "Cable" -in (aka female) connector on your video card, just like the kind of wire that you connect cable TV to your TV/VCR and your VCR to your TV. Most newer graphics cards have the capability to send the signal OUT to a TV, but few all a signal to be sent IN from a TV.
If you KNOW that you don't have one of these then check out ATI's line of "All-in-Wonder" graphics cards, or "TV Tuner" cards. They run as low as $70, I believe. Such as this one (), or this one ().
After you get this PCI card installed inside your computer, simply plug the cable wire from the VCR to your computer's brand new PCI card. The PCI card should have come with some video capturing software, just follow the simple steps.
PocketMatrix has some great guides on how to, after you have captured the video, encode it into an optimum format for PocketPCs, if you want to do that, too.