Also if you don't already know you might wanna set your video so that it saves in HD and widescreen. If you don't know how look it up on youtube. There are hundreds of tutorials and it is very easy, however some of the websites were you download the HD profiles have been took down so if you can't find a download link for them I'll give you them if you want.
Or you can read my post (i mean this one, not the last one)
To have it be widescreen HD, in Movie Maker, go to the Tools menu, click Options and click the tab labeled Advanced, make sure the video format is NTSC and the Aspect Ratio is 16:9
I'm not sure if this part is true or not, but i think your video may not be HD if the video's resolution is too smal, if that's the case, then you'll have to settle for no HD
Also, i don't think HD is required to make a video loop