First try trashing the preferences exactly like this:-
Delete the User Preferences
You can resolve many issues by restoring Final Cut Express back to its original settings. This will not impact your project files, but you should verify your Scratch Disk location setting after doing this. To reset your Final Cut Express user preference settings to their original state, do the following:
In the Finder, go to ~/Library/Preferences
Note: The tilde (~) represents your Home folder.
Remove the "com.apple.finalcutexpress.plist" file from the Preferences folder.
Remove the "Final Cut Express User Data" folder from the Preferences folder.
If that doesn't work you must reinstall BUT IT IS VITAL YOU COMPLETELY UNINSTALL FIRST like this:-
http://docs.info.apple.com/article.html?artnum=301182