Not genuine version of XP Pro

Hello,
I am a senior citizen with minimal tech knowledge. Two years ago my PC crashed. A friend took it to someone who told me that I needed a new harddrive. He installed it and installed XP Pro. It turns out it is not genuine.I had Xp home before the crash, and Microsoft had sent me a new disk, but I never used it, since the tech installed Pro. He never returned my recovery disks, plus the harddrive was no longer the original. There are things I cannot do, because my version of XP Pro cannot be validated. How can I replace the Pro by the Home version? I have a certificate of authenticity for the Home edition on the side of my PC.
Please, I need instructions for dummies!! Thank you for any help you might be willing to give me.
Irene