Why does Xcode 4.1 Installer ask me to quit iTunes if iTunes isn't running?
This is a widely-reported bug in some App Store installers. It sees the iTunesHelper process and displays this error, even though that process is not controlled by the user.

You can open Activity Monitor (in /Applications/Utilities), find the process and kill it with the Quit Process button in the top-left corner.



You can also instead open Terminal (also in /Applications/Utilities), enter the following line and press enter to kill it.

killall iTunesHelper