How to Upgrade the Windows 7 RC to RTM (Final Release)
 

Windows 7 checks whether or not the current version you are running is a pre-release copy, and prevents you from upgrading further. The solution is to edit a file inside the Windows 7 DVD—which you’ll have to extract to the hard drive to proceed.

http://www.howtogeek.com/howto/3075/...s-7-rc-to-rtm/