Product Key for Windows 7?

I bought my laptop from best buy about 2 years ago. It is saying that i need to re-activate my windows by entering the product key. I cannot find out where my product key is. I searched on the internet and it is saying the product key is on a sticker that came with the windows cd. Well when i bought the computer windows was already installed. If anyone can help I would greatly appreciate it. Also i have a compac windows 7 intel celeron laptop. Thanks