as you probably read, Apple will not be fixing QuickTime security issues on Windows anymore. Therefore we highly recommend that you uninstall QuickTime on Windows immediately. Of course you should check beforehand, if all your applications run well without QuickTime.
Good news are:
QuarkXPress 2015 does not have any dependencies on QuickTime, therefore you can safely uninstall QuickTime when using QX2015 on Windows.
Before you uninstall QuickTime on Windows, please first check your other applications, like Adobe's, Microsoft's etc., whether they don't have a dependency on QuickTime.
If you are using QuarkXPress 9 or 10 on Windows and have uninstalled QuickTime, then you might experience some issues when you use interactive functionality, e.g. for apps and ebooks.
So if you can't uninstall QuickTime on Windows, probably best advice is to isolate your installation in a virtual machine and/or disconnect it from the internet.