Search for "flv convert" and you'll find tons of stuff - and many freeware.
Make sure you update frequently as there seem to be new codecs arriving all the time (I use Ultra mp4 video converter - paid for but converts a lot of stuff both ways and lifetime upgrades).
As said, most flvs tend to be rather poor quality (Youtube are flvs) so using the disc space for an avi won't help (unless you have tons of room (I do) and happen to like using one format for everything - keep it simple, for a simple person is my motto ;D ).