Yes ,it's true many free converters :)
SUPER as mentioned from http://www.erightsoft.com/SUPER.html is quite good but needs some
understanding of bitrates etc .The reason why the author made it so hard to find the download link
is to make you read through all the pages ,so as to understand what is does.
But here are the direct links (to last page and download_
http://www.erightsoft.com/S6Kg1.html
http://www.erightsoft.info/GetFile.php?SUPERsetup.exe
at time of writing SUPER©.v2010.build.42(Nov.7,2010)
As to Format Factory -also mentioned before - be aware that it installs a toolbar.
Otherwise a good converter. Just uninstall toolbar if not wanted.
QuickMediaConverter http://www.cocoonsoftware.com/ is a bit simpler to use.
(Used to have donation nag ,not sure it still does ,but no trouble ,just ignore)
Since you need to convert to WMV format for WinMoviemaker,
you could also try this one http://www.wmvconverter.org/ (also portable version) presently v3.2
DL Links http://www.wmvconverter.org/download/wmvconverter_setup.exe
http://www.wmvconverter.org/download/wmvconverter_portable.exe
I have several more links incl the ones mentioned in other posts ,but this should do you for now :)
PS -Be aware that not every converter will convert to WMV format that Windows Movie Maker will accept.IOW, while the converted file will be .WMV ,it may not play or be joinable with other .WMV files.The reason is the the beginning or header may not be exactly what Windows MovieMaker requires.So you may have to experiment with the files and possibly different converters as well.
IOW, there is no absolute guarantee that every converted file will be accepted by WMM.
I know it's more than you wanted to know, but,
Thought you should know and be prepared for it ;)