If you have Yellow Mug Software's SizzlingKeys, a third-party application, installed on your computer, the application will prevent iTunes 5 from installing. Instead, you'll get this message, even if iTunes is not open:
Please quit iTunes before beginning this update.
To install iTunes 5, download and install iTunes 5.0.1 or later.
Important: Information about products not manufactured by Apple is provided for information purposes only and does not constitute Apple's recommendation or endorsement. Please contact the vendor for additional information.