you would need to right click your sound icon in the taskbar. go to playback devices. set optical output as your default device. push configure, check that its set to 5.1.
if both your soundcard and the home theater support the right formats then that is all you should have to do.
(if they dont, and you are only getting 2.0 sound.... then that requires some more effort..... but try the optical cable and settings first.)