Thanks for all your help/advice guys.
Im going to try that link that davieg has supplied to see if that works when i get home.
Just FYI, its the OEM version that was installed on it by overclockers and after doing a little more digging last night it says that the license is already being used.
Also, i've had a look in all the boxes and all over the tower and cant find the usual windows license sticker anywhere.