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Trashing iMovie's preference file will often fix problems like this.
Go to your User folder Library/Preferences (not the Library folder at the top level in Macintosh HD). If you are on Lion (Mac OS X 10.7) or Mountain Lion (10.8), the User Library folder is hidden. To access it, click on Go in the Finder menu bar, then press the Option key - the Library folder will appear in the drop down list.
In the Library/Preferences folder, locate a file named "com.apple.iMovieApp.plist". With iMovie closed, drag this file to the Desktop. When next opened, iMovie will create a fresh preference file with the default settings. If iMovie now runs OK, you can change these settings in the menu item "IMovie>Preferences" if desired. Also, it's now safe to drag the old preference file to the Trash. Trashing the file will not harm your existing Events or Projects.
Then try opening iMovie and try creating your video again...
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