"When I bought the laptop no DVD came with it, as it is pre-installed. So I'm a bit concerned because I think the guy installed on it a pirated copy (which I payed for it the full Windows 8 full price) and I'm not sure what to do."
Pre-installation is standard now so that alone is no reason to suspect software piracy. You do need to activate a new load of Windows within a month of buying it so you should not ignore it. You should go ahead and activate it.
Because Windows is pre-installed you should also run the utility which comes with your machine to burn a set of recovery disks. If you do not do this you will have no means of recovering your system if and when your hard drive fails or you suffer somemakor system corruption/infection.