Do you still have QuickTime installed on your Windows computer? You need to uninstall it. Even the Department of Homeland Security is getting in on the bandwagon suggesting users uninstall QuickTime from Windows systems.
Apple’s last update for QuickTime for Windows machines was released January 2016. On Monday, Apple confirmed with the Wall Street Journal that support for Quicktime on Windows machines has reached end of life. Since then several reports from various security firms has identified vulnerabilities in QuickTime. These vulnerabilities can be used to remotely compromise the system. For details on how to uninstall QuickTime for Windows please see Apple’s has published a guide which can be found here.
As for users that still have QuickTime on a Mac, you are currently still in the clear. Apple has NOT ceased releasing updates for QuickTime on a Mac.