Yes you get one when you take out a subscription with the cable supplier - usually Virgin Media or Talk Talk (was Tiscali) in the UK. VM also supply you with the cable box as part of their subscription.
The cards supplied for Virgin Media are only intended to be installed, and work, in their own equipment (set top boxes). It is outside their terms and conditions for anything else, including PCs, to be connected to the network.
Again I think there may be some confusion here . If the OP has a V or V+ box with a valid subscription then he can view the output of the box on his PC using a video capture card such as the Hauppage. However he could use a USB capture device also . The best of these are marketed under the Canopus brand but they are expensive. Otherwise I would recommend the Dazzle products as being good value for money.