Go ahead and find the clip, or clips, in your Windows Movie Maker project that you want to slow down go ahead and place them in the Timeline. You are going to want to use the Timeline view in this instead of the Storyboard view. Right click the clip and select Video Effects from the pop up menu. The Video Effects menu will come up with the available video effects in the left hand column and those on the selected clip in the right hand column. Go down in the left hand column and find Slow Down, Half, which will be under Sepia Tone and above Smudge Stick. Go ahead and select Slow Down, Half and then hit the Add button in the middle to bring it over to the right hand column. Once Slow Down, Half is in the right hand column hit OK at the bottom of the Video Effects window and it will be added to your Windows Movie Maker video clip.