For Flash Pro CS6/CS5.5:
Use keyboard shortcut keys to delete user configuration and preferences files. Hold down Shift+Ctrl+Alt (Win) or Shift+Command+Option (Mac) prior to launching Flash Professional. If this interferes with an OS dialog, hold this keyboard combination immediately after launching Flash until you see the prompt to reset preferences.
For Flash CS5 and earlier:
Delete user configuration:
Recreate Flash Professional Preferences