You prob need to update the product key. After installing it auto puts a key there, and says "activate now" or whatever, but isn't the key you actually use.
Go to Computer (or "my computer", whatever) and at the top it should say "organize, views, system properties.." etc. Click system properties, and at the bottom hit "change product key" and put the one you got in your box.
I know with mine when i installed the key there was not my actual key, i had to retype mine and then activate.