Problems with rendering a Windows Movie Maker video
This assumes that all the material, all the video clips are on your local disk and your telling Windows Movie Maker to save these in the same folder. Do all the clips have a suffix that is acceptable to Windows Movie Maker (see below for a complete list)? e.g. are these all .avi files?

How many clips are in the video? One or more clips may be at issue. To isolate, first save your project as it is, then remove one clip and save this as minus one. Test? if there is no change then remove another clip, save as minus 2. Test? If there is still issues then proceed to remove each clip one at a time and save each of these versions until you find the clip(s) that are having an issue.

Once you locate the offending clip(s) take these to another computer, and see if you get the same results. Report back.

MORE info from Microsoft:

You can import files with the following file name extensions into Windows Movie Maker to use in your project:

Video files: .asf, .avi, dvr-ms, .m1v, .mp2, .mp2v, .mpe, .mpeg, .mpg, .mpv2, .wm, and .wmv

Audio files: .aif, .aifc, .aiff, .asf, .au, .mp2, .mp3, .mpa, .snd, .wav, and .wma

Picture files: .bmp, .dib, .emf, .gif, .jfif, .jpe, .jpeg, .jpg, .png, .tif, .tiff, and .wmf