If you purchased a Windows disc through legal channels you should have the key printed on the disc or whatever documentation is with it and as far as I know if you purchased a computer with Windows pre-installed (through legal channels) the key is required to be on a sticker attached to the computer.
So, Fear, using software to retrieve that same key from a running OS that you purchased sounds sketchy enough to you to justify deleting my entire post? Also it makes the thread a little confusing. Not that any of the threads on here are far from nonsense, but still.
If you have a key and you're using Windows 7, go to control panel, click system and try re-entering it as I posted before.