How to shrink a pre-allocated vmdk?

I created a Win2k3 server with a pre-allocated 40GB SCSI partition on a 250GB HD. When I boot into Win2k3 and open up VM Tools, the Shrink tab says it is disabled for pre-allocated disks (and other factors).

I am only using 15GB out of the 40 and would like to shrink the disk down to 20GB.

How can I do this? Thanks!