Codecs, my friend, are programmes used in the COmpression and DECompression (notice the capital letters? That's where the term codec comes from) of audio and/or video files.
Sadly, it doesn't matter what codecs you install on your computer you still wont be able to import your .3g2 files into Windows Movie Maker.
The following are the supported file types of Windows Movie Maker:
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
Video files: .asf, .avi, .m1v, .mp2, .mp2v, .mpe, .mpeg, .mpg, .mpv2, .wm, and .wmv
This means that you'll need to convert your .3g2 files into one of those above.
One FREE programme that can successfully convert multiple audio, video, and picture files is called FormatFactory. You can download it from the following website.
Install the programme, and then you'll be able to convert away to your heart's content.
Happy editing, my friend.