Trashing iMovie's preference file will often fix problems.
Firstly, go to your Home (your user name) /Library/Preferences folder (NOT the Library folder at the top level in Macintosh HD).
As you appear to be on Lion (Mac OS X 10.7.5), the User Library folder is hidden (as it is on Mountain Lion - Mac OS X 10.8.2). To access the Library folder, click on Go in the Finder menu bar, then press and hold down the Option key - the Library folder will appear in the drop down list.
In your Home/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.