I’ve seen this a lot at our service desk, A student will bring in their laptop because they can’t open the Accounts prefpane.
After investigation I found there to be multiple guest accounts on the computer, one named Guest and the rest Guest1, Guest2 etc. These can be removed using dscl within terminal with the following commands.
This will show all the accounts that are on the system, you will need to find how many guest accounts have been created and remove them with the line command below…
That command will need to be repeated for each additional guest account.
Once that is complete you can open System Preferences and the Accounts prefpane should load correctly.
In Lion you can open Directory utility, located in /System/Library/CoreServices/ and use the directory editor within to remove the accounts if you aren’t comfortable with Terminal.