EDIT (April 23, 2016): The DEP problem returned. I have no idea why. The search for a solution continues.
One possible cause of the "iTunes was closed" data execution prevention/protection error is QuickTime's file association with certain file types.
One possible cause of the "iTunes was closed" data execution prevention/protection error is QuickTime's file association with certain file types.
After dealing with the problem and googling for solutions for years -- including updating iTunes and QuickTime -- I come across the following information recently at https://answers.microsoft.com/en-us/windows/forum/windows_vista-windows_programs/how-to-fix-itunes-problem-because-of-dep/1b3cf611-ed15-4886-a176-3867e5f542a4?auth=1.
Some of the users in that posting say repairing QuickTime fixes the issue. Since I reinstalled QuickTime already, repairing it seems silly. Others suggest uninstalling QuickTime. I do not do this but that should work if the issue really is due to QuickTime owning certain file associations. Finally, you can exclude iTunes from DEP. Users report varying levels of success with that last approach.
My iTunes DEP issue is happening on my Windows Vista machine. The way I make the problem go away is to open Control Panel -> Programs -> Default Programs [Set your default programs], and go through all the media programs and take away permission from QuickTime. For me, the change involves assigning "AIFF Format Sound" files (.aif, .aifc, .aiff) to Windows Media Player instead of QuickTime Player.