With the arrival of the recent iTunes 9 it is quite possible that some of the plugins, visualizations or add-ons will not work for you. What's more, it is possible until at some point it complicates the proper functioning of iTunes.
If we have any of these problems it is advisable to start iTunes in safe mode and then try to identify the cause of our problems and disable the add-on that causes the disorder in iTunes.
To start iTunes in safe mode, all you have to do is press Command and Option when we open it. As simple as that.
Source | Mac OS X Hints