[ubuntu] How do I reduce the size of a VDI?

I have a Windows guest that is currently 30GB and I have reduced the amount of space it uses within the guest (it's XP). It would be better if it was 15 GB. The original is dynamically allocated and I tried doing:

VBoxManage modifyhd "XP Plain-desk1.vdi --resize 15000

VBoxManage: error: Resize hard disk operation for this format is not implemented yet!

I made a clone with Virtual Disk Manager and changed the allocation to standard, and still get this error.

Is there a way to shrink this?