Cannot update or uninstall Acrobat 9 Pro

I have installed Acrobat 9.0.0 Pro on a MacBook Pro 13" with OS X 10.5.7. Several attempts to update it to 9.1.2 have failed, with the last producing a mesage that the application has been modified. The uninstaller could not select the application either (with a message that it has been modified). This has never happened before to me (Acrobat 4, 5, 7, and 8). Please help.

Thank you.