How can I put music on Windows Movie Maker?

I sometimes make slideshows on windows movie maker but I have found I can only put music on from the 'sample tracks' folder on my computer which really sucks because they're stupid songs! I have been told I have to convert them to WMA but I don't know how to do that! I don't want any download things that'll put viruses on my computer! Any help would be greatly appreciated, thanks! :-)