How to Edit movies in Movie Maker Windows
Make your movie look the way you want it to with the editing tools in Movie Maker.
Trim your video
You can trim the beginning and end of a video so your movie shows only the part of the video you want. For example, if you have a video with a few black frames at the beginning, you could trim the beginning of the video so the black frames don't appear in your final saved movie.
When you trim a video file in Movie Maker, the original video file isn't affected. All the video footage remains in the original video file.
To trim a video in Movie Maker
On the Home tab, in the Add group, click Add videos and photos.
In the Add Videos and Photos dialog box, open the folder that contains the photos or videos that you want to add, select the video files and photos, and then click Open.
Click the video that you want to trim.
To select multiple photos and videos, press and hold the Ctrl key, and then click each photo and video you want.
Under Video Tools, on the Edit tab, in the Editing group, do one or more of the following:
To set a new start point for the video, drag the slider underneath the preview monitor to the point that you want the video to start, and then click Set start point.
To set a new end point for the video, drag the slider underneath the preview monitor to the point that you want the video to stop playing, and then click Set end point.
To clear any new start and end points in Movie Maker so the whole video plays
Click a trimmed video in your project.
Under Video Tools, on the Edit tab, in the Editing group, click Trim tool.
Do one or more of the following:
To make the trimmed video play from the beginning again, click the start trim handle underneath the preview monitor, and drag it to the beginning of the video.
To make the trimmed video play to the end again, click the end trim handle underneath the preview monitor, and drag it to the end of the video.
On the Trim tab, in the Trim group, click Save trim to save the changes you made and to close the trim tool.
To quickly trim a video in Movie Maker, play the video you want to trim, and then press the I key to make the current point the start point, or press the O key to make it the end point.
If you trim part of a video that has a caption, the caption is trimmed too.
Split a video
You can split a video into two smaller items and then continue editing. For example, after splitting a video, you can move one video in front of the other to change the order in which the videos play in your movie.
To split a video into two items
Click your movie, and then drag the playback indicator to the point where you want to split the video.
Under Video Tools, on the Edit tab, in the Editing group, click Split.
Speed up or slow down a video
You can change the speed of your video in Movie Maker to make the it play faster or slower.
Click the video, and under Video Tools, on the Edit tab, in the Adjust group, click the Speed list, and then click a speed (depending on how much you want to speed the video up or slow it down).
Choose a theme
A Movie Maker AutoMovie theme will automatically add titles, credits, transitions, and effects to your movie. Preview the AutoMovie themes by pointing to each one with your cursor.
Once you've added the photos, videos, and music you want to use to MovieMaker, on the Home tab, in the AutoMovie themes group, click the theme that works best for your movie. You can continue editing, or just save your movie.
AutoMovie creates credits for your movie based on your tags. If your tags aren't correct, you can correct them by typing in new ones.
source : http://windows.microsoft.com/en-us/windows-live/movie-maker-trim-video-how-faq