Here is the issue: Client's PC had Win XP Pro on it when Hard drive went bad. Bought new drive for replacement and noticed the sticker (COA) said Windows 7 Pro OA. So I said to myself, self, let's just install Windows 7 Pro 64 Bit since I had a cd of it. Installed fine and went to activate it and comes up key is not valid??? What the heck. Doubled check to make sure all was correct and was. According to everything I have read, you should be able to use a different cd, but, of the sameOS type. Am I missing something here. The clients don't have any CD's for the OS.