Even maxing out virtual memory at 997 MB, I get insufficient memory messages. I have tried rendering in both form. Z and in the Imager Program and still no luck.
Soft shadows require memory to create a shadow map.
Turn on Estimate Memory Requirements in your RenderZone options. Turn off all but one light. Turn on a couple of more lights and notice how the meory usage increases. At some point you will reach your memory limit. Soft shadow casting point lights can eat up quite a bit of memory so if you have them you may consider hard raytraced.