Ran into an issue today on one of the Macs where we could not install Xcode because it wanted us to close iTunes. The problem was iTunes was already quit and there was no way to force the Xcode installer to continue.
Turns out the problem was a little process called iTuneshelper. To close this program and allow Xcode to complete do the following:
Open Applications -> Utilities -> Activity Monitor
Look for the Process called iTunesHelper and quit it.
Your Xcode install should now continue.
Bye or Now