Q: 64 bit itunes

You probably have a plug-in (most likely a visualizer) that's 32 bit. I used to use one called "Fountain Music" that was 32 bit and I had to do away with it because of this error.

Are you really running Mac OS X 10.5.4?

Apple menu > Software update.

Same problem but something strange just happened. After trying 3 or 4 possible solutions for hours I had just about given up. I downloaded itunes 11.1.5 for one last time but from a different server. No luck with Firefox or Chrome, but with Safari I had no problem. All started right up... don't ask me how. All is running beautifully, itunes 11.1.5 on OS X 10.9.2 Mavericks. Just thought I would let you know even though I know it is more luck than a solution.