Erroneous "Installation Alert" upon installing Xcode 3.2.6 I am on a clean Mac OS X 10.6.8 machine (I erased the hard drive and reinstalled the OS yesterday). I do have to re-download a few apps, and Xcode happened to be one of the first.
I completed the configuration and it started the actual installing . It said 'estimated time: 1 hour' but when I checked after twenty minutes it was almost ready, though it had stopped. I was greeted with this:

OK, an informative error message. Just close iTunes and complete the installation, right? Wrong, iTunes wasn't running. (I do know the difference between closing a window and quitting an app.) So I tried opening and quitting iTunes, but that didn't work either. Then I had a look at the Activity Monitor, but I couldn't find a process from iTunes. What do I do now? Even the buttons to cancel the installation are greyed out, so I don't even know how to retry the installation.
PS: Clicking on the find symbol next to iTunes in the alert screen just starts iTunes.
Update
I found a process called 'iTunes Helper'. I assume that's what's causing the alert. Is it safe to just kill this process?
And I found this: http://osxdaily.com/2011/08/02/cant-install-xcode-because-itunes-fix/! That fixes it, restarting the computer will relaunch iTunesHelper. That should have fixed it, according to the linked article, but sadly, I still have the alert opened, the installation doesn't progress.
OK, I solved it. So I first found this article, which said to killall iTunesHelper. That didn't work. After that I found an Apple discussions thread on this where someone had to killall AlertAll (the process responsible for the Install Alert). I did that and the installation finished succesfully.
Step-by-step

Go through the installation and get this alert
Open Terminal and execute killall iTunesHelper (or kill it with Activity Monitor)
Execute killall AlertAll if the installation still doesn't proceed (Caution: after this the installer assumes iTunes is completely stopped. If it's not, things can of course go wrong. So better look over your processes list twice before doing this.)
Everything should be fine

This was exactly what I did, if it doesn't work for you: I'm sorry, but then your issue is different and you should ask a new question.