sky on pc help

My Location
Hi all, I hav just had Sky+ fitted and had my old amstad drx100 box fitted next 2 my computer. I hav bought a visionDTV mini ter tuner card.
I get all the free view channels if I connect to my antenna, but if I plug my amstrad in it picks nothing up. If I plug in the antenna in the sky box it picks up the free view but no sky channel (with viewing card in ) I have check the output on the Amstad straight to a tv and I pick up all the sky channels.Is there a reason y I can not pick up sky channels from the sky box ????.
Has anyone got any idea's.