There different way you can find out the cd key
1. If you explore the windows cd and open up I386 it does reside in a file named UNATTEND.txt. I have successfully used this on VALID windows installations
2.The product key is typically printed on a label on the product packaging containing the CD when you purchase it. If you look closely at several copies of the same product, you'll see that the key is different on each label.
3.The product key is typically printed on a label at the bottom side of your laptop.
4.The product key is typically printed on a label at the right side of your desktop Cabinet.
Cheers
