If the version you have installed (Home Premium/Professional/Ultimate) is the same version that you are going to pay for then you should be able to change the product key on the Control Panel somewhere.
Just installed windows 7 on my PC, looking pretty swish if I'm honest. Had XP and a couple of shots on Vista but the college allowed us to download Windows 7 professional for free so i gladly took it haha!
If the version you have installed (Home Premium/Professional/Ultimate) is the same version that you are going to pay for then you should be able to change the product key on the Control Panel somewhere.
since windows 7 was my idea then I say how dare u get a counterfit copy! Jokes aside Stu Ws seems to be the best solution. Try registering a new product key without reinstall but if that doesnt work then do Stus idea