I've moved this so you'll (hopefully) get a better response hunter.

I remember this question was discussed before at Moviekids (You need to Login or Register to view media files and links)
maybe that will help you. Otherwise it appears we'd need to know more about what sort of software you have before anyone can help.

"From TV you need a TV Tuner Card installed in your computer."

That's not quite true. As long as you have a regular video input on your video card, you can always pass the signal through a VCR. I found I always got better results that way than with the TV tuner.

Basically, there are dozens of different ways you can do caps from different media. It all depends on what hardware and software you have, as well as personal preference. A lot of media players have built in still capture options, which is probably the easiest method if you just want to get a few caps of specific moments.