If you jumped onto Microsoft Windows 7 pre-release version (Beta or RC) you can't upgrade to the final RTM version and need to do an clean install loosing all your settings and installed application as Windows 7 setup won't let you upgrade from a pre-release version to final build displaying error as below.

Luckily there is a simple config-file hack which allows you to do an in-place upgrade from Windows 7 pre-release to final version keeping your files, settings and applications intact.