QuickTime may be corrupted. You must reinstall before iTunes will work. Windows users, also try disabling the compatibility mode of QuickTime.
Right click on "C: \ Program Files \ QuickTime \ QuickTimePlayer.exe" ("Program Files (x86)" for 64-bit Windows) and choose Properties from the context menu.
In the Properties window, click the Compatibility tab. Uncheck "Run this program in compatibility mode for".
Click Apply.
Close the Properties window