Bmibaby allow laptops as extra luggage on top of its allowance. Ryanair only allows it in its 15 kg. Easy jet in its 20kg. And then all depends on the airport and the smiling girl/boy it seems behind the desk.....
Put it in a plastic carrier bag. Even if you wander up to check-in with ten loaded carrier bags, they won't say a thing. Airlines do not count plastic carrier bags as part of your luggage - honest! (But don't spread the word, or they might just change the rules!)