I needed to install a new hard drive in a computer, and I used my Windows 7 cd to install the OS, however, when I got to the Product key, I used the correct one but it won't accept it. I found that that can happen if you install from an upgrade disk, which I did (I thought it was the full program), and it said that I should upgrade from XP or Vista. I have my old XP disk, but when I reboot from the CD I get the Blue Screen of Death. If I restart normally, Win 7 is still there, but what will happen if the product key doesn't work? Will I only be able to use it temporarily? What can I do???