Bought a laptop from ebay and now validation key is blocked?

About 2 year ago, i bought a laptop from the famous ebay.I'm not sure if i done something to it (probably did but don't remember) or if the person i got it from done something to it.so now i have to put up with a sign in the bottom corner saying "You may be a victim of software counterfeiting. This copy of Windows did not p genuine Windows validation" really annoying. so i looked at the bottom of my laptop for the key, i entered it but it doesn't work.and i don't exactly have �100 odd quid to get a new key.But the other day i wanted to upgrade my Windows media player but i got a notice saying "This copy of Windows did not p genuine validation. The product key found on this computer is a Volume License Key that has been blocked".what am i to do now? Do i need to buy a new key or is there a way to unblock it? And i haven't shared my key with anyone else.