Starting applications (or your OS) in Safe Mode prevents additional applications and add-ons from loading. In the case of starting iTunes in Safe Mode, all those extra visualizers and device plug-ins will be disabled, allowing for a more thorough troubleshooting session.
To load iTunes 9 in Safe Mode, simply press and hold (Command + Option) and click to open iTunes 9. Continue holding the Command and Option keys until you see the iTunes Safe Mode warning:
For more on iTunes tips and troubleshooting, check out these MacFixIt articles: