Installed OEM Win 7 in Bootcamp

I have mistakenly installed an OEM version of Win 7 (from an old PC that is no longer used) in bootcamp and the license key doesn't work. It never even asked me for licence key when installing but after spending days installing all my other windows software and setting up the machine I see that the OEM key that it must have got from the installation disks doesn't work and it is in trial mode which will expire soon.

What are my options to avoid having to start from scratch, if I purchase a full license can I simply insert the licence key without having to re-install anything?

I also have a Windows 7 upgrade disc and I have read that can be used to install without entering key and then re-installing ocer it with key? Will this work in my situation?

Help appreciated.

 

Thanks guys, good to hear I don't need to re-install, I'll stick the upgrade disc in and see what happens on the minimal off chance it may work. However after the 120 bucks I already coughed up for the upgrade version of Win 7 a few years back don't think I can stomach coughing up another 200 bucks for an operating system I'm trying to get away from........(just converted to Mac) so I will probably end up removing windows from the mac!