Microsoft just opened a link to download Windows 7 Release Candidate (RC), that means the OS passed beta and it should be stable now. You can download the ISO image and product serial number here:

http://www.microsoft.com/windows/windows-7/download.aspx

However, if you have installed the old Windows 7 build 7000 open beta, the RC didn't allow you to upgrade from old pre-released Windows 7 build and it suggest you to make a clean install.

But it is troublesome to do clean install during each upgrades. Below is a quick guide on how to skip the system checking and allowing you to install Windows 7 RC version over the old beta. It is highly recommended that you should backup your files before the upgrade.

1. Copy the Windows 7 RC DVD to harddisk.
2. In the "sources" folder, you should find a file called cversion.ini (or cversion if your file extension is hidden). Right click and select edit, to edit the file in notepad.
3. You should see a file "MinClient=7077.0", change it to "MinClient=7000.0" so that the system checking allows you to upgrade from build 7000.
4. Run the setup.

Reference: http://www.neowin.net/news/main/09/04/07/windows-7-beta-to-rc-transition...