Product key. I had my computer repaired at bestbuy my computer had a virus so i reboot and put my

Question

I had my computer repaired at bestbuy my computer had a virus so i reboot and put my windows xp software into my computer and the product key your store gave me they said it had been used before and i cant activate my software on my computer i have had this computer for over 5yrs and i have been the only one using i dont share my software with anyone. I really want to know what bestbuy going to do about this. Thankyou for fixing my computer its been better but now i cant activate my software. from eddie