Files compatible with Windows Movie Maker include:
.asf, .avi, dvr-ms, .m1v, .mp2, .mp2v, .mpe, .mpeg, .mpg, .mpv2, .wm, .wmv
Google: free video converters online and download one. Convert the 5g videos to one of the file name extensions (formats) listed above. If the conversion is successful, you will be able to import the iPod video into Windows Movie Maker.