Hello Nic Lit,
To activate Windows 7 Professional, make sure you have a genuine key for the Pro edition. Connect the PC to the internet, press Win+R, type slui 3.
Then, just enter your Windows 7 Professional key and follow the prompts. If online activation doesn’t go through or you’re offline, press Win+R again, run slui 4, choose your country, and complete the automated phone activation using the Installation ID shown on your screen and the Confirmation ID provided by the system.
I hope this helps.
Regards,
Finn.