Fistly I applogise for misleading you. I though the cable you where using was composite to be digitised by the PC.
SKY adds it's own channel to the channels picked up from the aerial. Note channel, not channels. With SKY off the TV tuner card in the PC will still see the normal channels from the aerial.
You need your sky on a channel that's clearly sky and cannot be mistaken for a TV broadcast. With SKY on you need to redo the channel search using the TV tuner in the PC. It will find one, note one, new channel, the one SKY is using.
Any program you watch on SKY will now appear on the one SKY channel on PC Tv tuner.