OK – this is broken. The version of QuickTime that comes with the latest iTunes download is conflicting with MS06-15 (kb908531). The only work around seems to be uninstalling the hotfix (which is a critical, remote exploitable one – so don’t do that).
You can read the whole thread here.
Some guy got it working using these steps:
– install iTunes 6.0.5
– get the stand-alone installer for QuickTime 7.1, start it
– go to the temp directory and copy QuickTime.msi
– right klick on QuickTime.msi and choose uninstall
– install QuickTime 7.0.4 from the link above
– use iTunes 6.0.5….
[UPDATE: The above procedure worked for me, too]