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.