Windows 7 64 bit
i5 2 core laptop
6gb memory
GT620M
blender-2.70-ba1acec-win64
so the problem is pretty simple to explain
here we have AO Pass from cycles.
http://fc08.deviantart.net/fs70/f/2014/144/c/3/ao_pass0026_by_kr4mon-d7jjh20.jpg
and as u can see the domain from the smoke, is rendered as a cube.. sort of..
And here is render without AO
http://fc00.deviantart.net/fs71/f/2014/144/5/b/smoke_by_kr4mon-d7jjgjh.jpg