What make is it? Vodaphone were giving them away last year with certain new laptop purchases. They came with SIM cards but they had to be registered on a PAYG tarrif within a certain time period to activate the SIM.
If you have the original packaging and registration details it may still be possible. Think of it like a mobile phone (which effectively it is) - it is locked to the original manufacturer and fairly useless unless you register it with it`s own network.
One word of warning. On a PAYG tariff these are very expensive to use except in an emergency although they can be used at Wi-Fi hotspots once activated.