I was trying to update my Xcode from the Mac App Store and this got at “Installing .. 1 minute remaining”. Most of the internets is chockfull of information about running uninstall-devtools from /Library/Xcode. Since Xcode 4.3 is a completely packaged application, you can apparently just drag your Xcode into trash. If that doesn’t work, just do a sudo rm -rf /Applications/Xcode.app and it will remove the app for you. Restart the Mac App Store and you should be able to set Xcode up as a fresh install.