Currently Being ModeratedNov 4, 2012 1:55 PM (in response to Sejjady)
I think you are right. It is likely the disk space. You may consider moving some of your files to an external drive.
See this User Tip for how to move things in iMovie.
In your case, you would need a USB attached drive that is formatted as Mac OS Extended (Journaled). You can puchase a drive that is already formatted this way, or you can do it yourself using Disk Utility.