Usually you just need to change the product key. You don't have to re install anything,just right click on computer icon ,go to properties and on the bottom hit the "change product key" enter the valid product key and follow the prompts.
however you say you 'cannot now use it' ? Can you tell us more of the history so we can figure out whether it is just not Activated/not got a product key at all or if it is a Pirate version. And tell us what appears on the Monitor Screen now when you switch on the laptop.
or You may be able to do a Repair or an Upgrade install with a genuine DVD. Need more details from you please.