System restarts as soon you start any download

A peculiar problem has started in my system. As soon u commence any download the system restarts. but if you download anything in Opera web browser (in the active transfer window) the restart thing does not happens. My Quickheal version shows I worm. Brontok.a. Is this happening because of this or is it any other problem.can someone tell how to remove it. plz help fellas.