How can I unlock my 2nd hand digibox?

My original digi box packed up, so a friend gave me an old 1 thats no longer needed, which still works perfectly. Unfortunatly my friend didn't have sky sports or movies which I subscribe to, and when I try to watch either of these I get an onscreen message telling me that this is the wrong viewing card for this set top box and I should insert the correct viewing card for this box.
The box is a Pace, model no: BSKYB 2500B, part code:540E2304070.
Any ideas, or do I just shell out for a new box?