Have you tried the usual fixes for unresponsive software:
1. Move the iMovie preference file to Trash - a new one will be generated when next you open iMovie.
It is found in: User > Library > Preferences > com.apple.iMovie.plist
Trash the com.apple.iMovie.plist.
2. Repair disc permissions.
Open Utility > Disk Utility. Highlight 'Macintosh HD' in the left pane then click on Repair Disk Permissions.
It will take several minutes to do this.
Finally reboot your computer.
You could also try to re-install iMovie from your installation disc.
However I would seriously consider an upgrade to OSX 10.7.5 and iMovie 11.
If you are playing your project in the full screen viewer, it remembers the last moviie you were watching, and it plays that. You can drag your cursor to the lower third of the screen in the middle and you will see a screenflow like representation of all the projects and you can navigate to your current project and watch it.