When was the last time you defragged your HD? or is the free 5GB virgin? is it on your C: drive? Windows can only use unfragmented contiguous space on C: drive for virtual memory. Sometimes it is not the final render size that eats up the memory and scratch disk, it is the combined uncompressed texture sizes of all the figures in that scene plus shadow maps, trans maps, etc that are compiled during the render process (and then deleted when the final image is copied to the filename you specify. Other than that, if the problem persists, you might have found a bug that should be reported to Curious Labs.