Homeland Security issued a warning that Windows users should uninstall Apple Quicktime. Quicktime on Windows contains security holes that Apple will no longer fix because they have ended support for Quicktime on Windows.
Unpatched vulnerabilities in Quicktime could be used to compromise a Windows computer from another computer across the Internet. The program will continue to work, but creates serious risks. The only solution is to uninstall Quicktime from all Windows computers.
For more information on uninstalling Quicktime, see:
https://support.apple.com/en-us/HT205771
P.S.
Windows can play many movie formats without added software. If a player is needed, VLC Media Player is free, open source and recommended by many reviewers.