How can I install correct version of Win7 over a copy?

Hi Guys, I fell from grace when my computer got a really bad virus and had Win7 re-installed, and hastily. Problem was, I didn't check the credentials of the guys wj¡ho installed it, and it turns out it's a pirated copy. I really want to rectify this, as I don't ever inrtend to install Win8 or 10, and want to keep Win 7 Professional probably for ever. I'm down with the system, like it,...