Installer won't accept my admin password

For the past month or so, any time I've tried to go through the process of installing an app that requires Installer to do the work, when it comes time to enter my admin password, nothing happens. The password box pops up, I enter the correct password (as when I enter an incorrect one it does its "incorrect password shake-thing") and, like always, the admin password box disappears. Normally at this point, the installer continues on to install the app, but instead it does absolutely nothing. I can click "Install" again to enter the password, but, again, nothing happens. I've tried changing my password, deleting preference files, doing a disk repair and permissions repair - nothing.

I'd really like to not have to re-install Lion, but it looks like that might be what I need to do. Any input would be greatly appreciated.