http://www.wintips.org/fix-sysprep-not-able-validate-windows-installation/

HKEY_LOCAL_MACHINE\SYSTEM\Setup\

At the right pane, DELETE the Upgrade key.
Close Registry Editor and restart your computer.

Now run SYSPREP again. Sysprep should run successfully now and without problems.

• Note: Always run "SYSPREP" command as Administrator and uninstall any security software before running the command.