[fabienne de vries]"whenever I pre-render a composition, After Effects automatically replaces the composition with that mov. Why is it doing that and how can I turn it off because I don't like it :)"
That's exactly what pre-rendering [link] does -- it renders a composition and replaces its usage with the rendered output.
What are you trying to do? Perhaps you'd rather use a proxy [link] workflow? Although most folks think of proxies as low-resolution or low-quality versions of a comp, there's nothing to stop you from using high-quality renders as proxies to speed up your work.