Does Itunes and/or Bonjour show up in Control Panel / Add or Remove Programs?
If either are there, download and install Revo Uninstaller Free
Run it. It will show you the same installed programs you see in Add or Remove Programs.
Select Itunes then click Uninstall.
First step does the actual uninstall. If it works or fails, continue to the next step.
This step scans for leftover folders, files and registry entries. When complete, click Next.
This is the left over registry entries. Click the Select All button the Delete. Only the entries that are highlighted will be deleted, even though others are selected.
This is the left over folders and files. Click the Select All button the Delete. Any that cannot be deleted will be deleted the next time you reboot.
Repeat for Bonjour if it's still there.Note:
Be careful so you do not allow a reboot until everything is done, then reboot manually.
Revo creates a System Restore point before each uninstall and cleanup is started.
Reboot when done.
Try the Itunes install.