Help with Parallels Desktop 9 for Mac

Hi, I've recently purchased Parallels 9 for Mac to run a number of design programs that I need for uni that are only available on Windows OS. I understand that in order to use Parallels I need to purchase Windows 7 or another Windows program (if anyone has tips on which one please do tell) - I was wondering whether I need to buy Windows for Mac still or just regular Windows for PC? Please note that I am not talking about installing Office as I already have Office on another Mac and don't need to install on this one I will be using for these windows programs. I hope this makes some sense, thanks for the help in advance