Hey there, XxMmMxX.
From what I understand, you already have Windows 7 installed on a hard drive, which you've been using with a different computer and you want to use that hard drive with your new build, but please correct me if I'm wrong. If this is the case you won't be able to boot to Windows which has been pre-installed on the drive for a different system (OEM - Original equipment manufacturer). In this situation you'd need a Windows installation media (DVD/USB) in order for you to make a fresh install of Windows on your new drive. Note that I'd recommend that you have only the drive you want to install Windows to, connected during the installation process in order to avoid possible booting issues in the future.
Hope that help...