What you are trying to do requires the Volume license key. you cannot simply replace OEM key with MAK key. You have to reinstall your computers with Volume Licensing media. Then capture this image, then apply this image to your computers around your enterprise environment.
I have attached MS whitepaper docx for the Volume licensing an also, here is an article which instructs you how to legally modify and reimage an OEM copy :
http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=8f9753d6-0714-4940-884a-33027683e328
I have also Attached the OS License Q&A. Please refer and let us kno if you have any questions
Reagards,
Vikas