I may have missed something but if you have a coaxial cable between your sky box and the RF out connection on your TV card you should get whatever is showing on your sky box. I presume this what you are saying by "I've attached an aerial between the RF Out on the back of the Sky box to my PC TV card" Try the coaxial into another TV just to make sure that you get a picture through the coaxial.
Years ago you had an analogue TV card in the computer this allowed you to tune into the RF signal sent via the RF out of the sky box the same as tuning the old video recorder to a TV.
If I'm correct the RF out from your sky box is still an analogue signal. Your digital tuners in the computer cannot tune to an analogue signal only digital. Crossbow7 seems to have one solution there may be others but have not found them as yet.