When do you get this error? Are you trying to upgrade from an older version of Windows to Windows 7? Is it an old computer that you are trying to install with Windows 7? Are you going from 32bit OS to 64bit?
First and most common issue...BIOS is not up to date or its using a Beta version. Check with the manufacturer and update your bios to the latest non-beta version. DO NOT flash it unless you absolutely have to, if your currently bios is corrupted or will not allow you to downgrade out a of a beta version.