Solution: Could not load Users & Groups Preference Pane
Too late for you, but maybe not others:
From:
https://discussions.apple.com/message/15721474#15721474
Open: Mac HD/System/Library/Core Services/Directory Utility
Click the lock to authenticate changes
Click on Directory Editor
Click to highlight any extra Guest accounts ("Guest1" "Guest2" etc.)
Delete them with the (-) button, leaving only the standard "Guest" account.
Reboot
Command-Line method (useful for single user mode?)
Open Applications > Utilities > Terminal
Enter: sudo dscl . delete /Users/Guest1
Reboot
NOTE: This method may require repeating the procedure for Guest2, Guest3, etc. accounts.
If symptom persists or Disk Utility is crashing, consider this fix:
Copy "PlatformHardwareManagement.framework" from another (working) machine and place it in the same directory on your own upgraded machine. (File also available on Snow Leopard OS 10.6 Macs.
OR
Use the program "Pacifist" to open the Mac OS X Snow Leopard Install DVD (10.6)
Manually pull out the PlatformHardwareManagement.framework and replace the copy on the hard disk.