There are lots of movie to avi converters out there. Trouble is you will find that a lot of the free versions are trials that either don't save or do maybe 10 seconds. Then theres the ones full of adware, and also the fact that a lot of the "download" links take you to pages with a big "download" button that's related to an advert or some useless "let me tell you how many viruses you've got so you really need my c**pware" program.
Anyhow I've done a bit of a google and this lot
http://www.radgametools.com/ looked promising. There's a link on the page which takes you to a download page. I've just downloaded installed it on XP Home SP3, nothing horrible detected by AVG Free.
When you fire it up you get a windows explorer style "Select a file" dialog; the default action is to play the file but in the buttons at the bottom of the windows there's a convert option. I've converted the Quicktime sample movie to avi, seems OK. I just used the default settings.