While trying to install or uninstall QuickTime for Windows an error message appears, and the install/uninstall fails. The message may indicate that you must exit all programs using QuickTime.
Check to make sure no other applications that use QuickTime are open (including iTunes). If there are no other open applications, then make sure the QuickTime control panel window is closed to install or uninstall QuickTime. Close the Control Panel window, then try to install or uninstall again.
To uninstall some older versions of QuickTime, use the uninstall application available in the Programs menu under QuickTime.