Try deleting iMovie's preference file. The file is located in your user folder Library/Preferences, and for iMovie '11 is named "com.apple.iMovieApp.plist". If using Mac OS X Lion, to gain access to the Library folder, Option-click on Go in the Finder menu - the Library folder will appear in the list. As you are on Snow Leopard the Library folder will be accessible like any other folder (no need to Option-click).
Drag the .plist file to the Trash. Now try opening iMovie. If things are working OK, iMovie will create a fresh preference file with the default settings. These settings can be changed if desired by clicking on iMovie>Preferences in iMovie's menu.
Deleting the preference file will not harm your Events or Projects but may fix iMovie's odd behaviour. The file can become corrupted in some circumstances.
NOTE: Don't look for the .plist file in the top level Library folder (the root level). The file is in the "Home" Library, that is, your User folder. Also, if you are using an earlier version of iMovie, the preference file will have a similar name to that I've mentioned above - usually with the version number included in the name.
Message was edited by: John Cogdell - added last sentence under NOTE.