How do i convert a movie into a HD movie and keep it on AVI?

i have found a small episode of a movie on youtube and it is on high
definition i need it to create pictures for a GIF but that program
only works on AVI movies and i converted the movie into a AVI but it
lost allot of it's quality what can i do to fix it and keep the
quality of the movie high and also turn it into a avi?