Most video cards with TV out will put out composite or S-Video. You'll need a RF Modulator ($20 or so at anywhere that sells consumer electronics, especially gaming consoles). This will convert the signal from the video card into RF Channel 3 (or 4), and then all you need to do is tune the television to the appropriate channel.
Be aware that the picture won't be as good as if you were able to hook S-Video directly to the television input.