What's the tuner Dave ? and what system are you running, Vista or XP ?
Does the box say that the tuner is certified for your OS ?
I have an Ati AIW card which works in XP but not in Vista, and an Hauppauge Nova T-500 that works on both.
Does the tuner work with the software bundled with it ?
Did the drivers auto-install when you powered up after installing the card ?
If not, have you manually installed them ?
Does the card show in Device Manager without any warning symbols ?