A Sky box, without a
valid viewing card, will only receive the unencrypted 'Free to Air' channels. These include BBC1, BBC2 and ITV1 but not C4 and C5.
C4 and C5 are 'Free to View' channels, which means that they're encrypted but they don't require payment of a subscription. However, you do need a 'FreeSat' card, which you must purchase from Sky for a one-off payment of �20. My guess is that you've got an old Sky subscription card in the slot, rather than a FreeSat one, so you're only receiving FTA channels and not FTV ones.
To view a list of Free to Air channels, see those marked 'FTA' in this list:
http://www.wickonline.com/fta.htm
For a list of the channels you should receive with a FreeSat card, use the scroll bar here:
http://www.freesatfromsky.co.uk/?pID=3
(Ignore the reference, on that page, to a one-off payment of �150. That only applies to people who haven't already got a Sky dish and receiver).
To purchase a FreeSat card, phone Sky on 0870 240 5651.
Chris