I don't think Windows Movie Maker will rip a DVD...as for converting, MovieMaker doesn't (?) convert AVI packaged DivX movies does it? And of course...there's always the issue of flash .flv files.
MPEG2, which doesn't seem to be so popular anymore, is convertable by MovieMaker...as far as speed...probably depends more on your PC than on the software itself.
If MovieMaker meets your needs, then no need buy/dl any other software