The only way to determine if a Windows product key is genuine is to use it to install the Windows operating system, then visit Genuine Microsoft Software and click on "Validate Windows". If you purchased a Windows product key from a third-party source, without the installation disc, it's likely non-genuine.
Please see:
Don't fall victim to stand-alone COAs: http://www.microsoft.com/howtotell/content.aspx?pg=coa&displaylang=en
Moderator Edit: Please note below updated support content related to topic
Carey Frisch