Amazon games and software downloader has to be installed if you want to purchase any games or software from Amazon. This is required to downloaded the purchased product from Amazon. By default Amazon downloader automatically starts during Windows startup. But you can disable automatic restart using the Amazon Downloader options.
Right click the Amazon Downloader icon on the system tray and select Options menu from the menu list.
This would display the following Options window.
Under Startup section, un mark the check box with the label as Always restart the Amazon Games & Software Downloaded when rebooting. This would prevent Amazon downloader from restarting during windows startup.