My computer was a pre-built one from bestbuy that came with Windows 7, however some time has passed and its time to do some cleaning up. I want to do a completely clean install of windows 7, now since it was pre-built it did not come with a win7 disk, however on the side of the case there is a product key. I was wondering if I could download windows7 and use the product key on the side to activate it, or if it will tell me that it is invalid, or already used. Thanks for the help