The only thing I could suggest would be to remove all of Apple's software, including Bonjour, if previously installed, and the Mobile Device app. They should all be re-installed when you install iTunes 9.
Then, restart the computer before you install iTunes 9. There could be a memory resident driver or app that is causing the error in question, the restart will allow the resident driver or app to be uninstalled, allowing for a fresh install of the software.
I am assuming that it is a Windows PC that this is happening on, right? If so, what OS is it using?