help! fledgling animated icon maker seeks assistance!

(sorry about the long title!)

hi- i've been reading all these mini movie tuts
and they sound great...

but i have a problem...

all the video i want to turn into icons/use in general for animation shop/ir are .mp4

and they're not compatible with anything!

does anyone know a good video conversion tool i could get my hands on?
(shareware, trial whatever?)

any help would be very very very much appreciated!

thanks!

xxx