I've downloaded XCode 4 from the App(le) store and started installation. After approx. 3/4 of the installation process, an installation alert tells to close iTunes. But iTunes is not running and I never run in before after installing a fresh (!) OS X 10.7. When clicking the zoom icon in this alert, it opens iTunes. Closing it does not change anything - the installation does not continue. Rebooting also does not help. What else I can try?
Start Activity Monitor
Locate "iTunes Helper"
Kill it (Quit Process)
Continue installation of Xcode