1.Are you using Windows movie maker or Windows live movie maker?
If you are using Windows Live Movie Maker there is no Slow Down effect available.
Only Windows Movie Maker v2.6 has this effect. I suggest downloading Windows Movie Maker v2.6 if you want to access the 'Slow down, half' effect.
To make your video slower using this version of Windows Movie Maker, you need to use the "slow down, half" effect. On the storyboard/timeline, select the video clip, picture, or title to which you want to add the effect. Click 'Tools', and then click
'Effects'. In the 'Contents' pane, click the effect that you want to add (in this case, "slow down, half"). You can click 'Play' under the preview monitor to see a preview of what the effect looks like, or just click and drag the clip onto the video or picture.