I had a fruity Installer application. Not sure how it got messed up, but the result was the same as what you're talking about. I couldn't install iTunes because my installer app was mangled. So I replaced it with a copy from another machine, and that didn't make any difference till I restarted. Then it was all fine.
Try replacing your Installer.app and see what that does for you. I'd be all sorts of willing to spew some files to you if that's what it takes.
