I believe I have the same problem but when I try to delete the com.apple.HIToolbox.plist file I get the following message:
The operation can’t be completed because an unexpected error occurred (error code 100022).
To clarify, my problem is that I have lost my admin privelages dispite being logged in with the admin account.
At the password stage of installing any package, when I enter my password, the password the dialogue box will disapear but I will not have advanced a step in the package wizard.
Also, when trying to unlock the padlock in system preferences, once the password is entered, the dialogue dispaears and the padlock remains locked (but is highlighted like when you hover over it).
I know I'm entering the right password because if I enter any other password, I get the standard wrong password reaction. I am also definately an admin, it says so under my name in 'current user' sidebar of 'users and groups'.
I am running Mac OS X 10.7.2 on a 2.3Ghz Macbook Pro.
Any help is greatly appreciated that doesn't involve reinstalling the system.
Many thanks,
Dan.