If a previous version of MediaPortal is installed on your PC it will be uninstalled automatically.
If the installer needs to download Windows Media Player 11 or MS SQL Express, it will provide a link to download from Microsoft's website, and a location to save the file.
If using a 64 bit version of Windows and the installer needs MS SQL Express, you should rename the downloaded file from "sqlexpr.exe" (the name used by Microsoft) to "sqlexpr64.exe" (the name expected by MediaPortal), if the installer asks for that name.
If you are still using Windows XP, you will receive a message while installing MediaPortal, that your OS is not supported. After clicking OK, the installation process will start as normal. Remember that all functions of MediaPortal might not work when using Windows XP. Consider upgrading to a modern operating system.