Try leaving the single system partition of a given size in the image
and then set a post-sysprep task to run the DISKPART command-line disk
management utility to partition and assign drive letters to the
remaining partitions.
http://support.microsoft.com/kb/300415
As diskpart can be ultimately destructive, please test this on
machines that do not hold your "golden" image or you'll be banging
your head on your desk.
Jim