Windows Vista video files: Convert .MOV to .WMA?

Earlier this year, I somehow managed to change the format of videos that I upload/download to my computer from the Windows supported .WMA to the Apple format of .MOV. Now, I’m attempting to edit a video using Windows Movie Maker, but it does not support these .MOV files. Is there any way to change the format back to .WMA, or do I need to buy a program that will convert the files for me?