2010 MacBook: Can't unlock Preference panes
I have a mid-2010 MacBook running OSX 10.7.4. Computer's operation is normal except for this issue. The issue is that I cannot use the administrator password to unlock preference panes or install software, but it works when logging in. I have tried restarting, resetting PRAM, repairing and verifying volumes from the repair utility, etc. The Users and Groups preference pane is locked, so I cannot create a new administrator account. I have also tried changing the password. The computer occasionally reverts (seemingly at random) to the old login screen (two text boxes, one for username and one for password on a gray background), and on this screen I cannot login either. I would like to confirm that the username is correct when I enter it, so you know that isn't the issue. The password is definetely correct, as I mentioned earlier. When installing software, it recognizes that the password is correct/incorrect (when incorrect, the box shakes), but it just closes quickly, having decidedly not installed the software.
Thank you for any help!
MacBook, Mac OS X (10.7.4)