This error message means that the product key you entered (probably the one on the sticker that is stuck to your laptop or desktop PC) does not match the installation CD you are using.
Your PC probably did not come with a Windows 7 installation disk and your hard drive died so you are using a borrowed installer CD.
To get around this error you need to find the right installer CD for your exact copy of Windows so that if you have Window 7 Home Premium you cannot use the installer for Windows 7 Professional.