Is there any simple and free way to convert downloaded YouTube videos flv into aviDivX file?

yes!

OpenSource and free of charge solutions are:

* find and download freeware program called AviDemux ( homepage is http://fixounet.free.fr/avidemux/ or just google it )

* open downloaded flv (flash video) in it

* select video codec (MPEG-4 Xvid)

* audio codec (MP3)

* format (AVI)

* and then click on Save button ... easy

Paid or demo solutions are:

* i dont know...