you ask 'without having to use additional software' - that makes it more difficult.
you haven't given us much detail about your system, but most graphics cards drivers
allow you somehow to rotate the whole screen/desktop.
it will also rotate your taskbar and everything else, so you'll have to rotate it back to normal after viewing.
if you decided that you do need extra software after all, try the excellent (and free) avidemux
you'll need to use a 180 degree rotate filter on the whole movie, and it may take a while to process and re-transcode the whole thing...
post back if you need further help.