How To Shrink a Window’s Virtualbox VM

1. Perform a hard disk defragmentation (windows)
2. Download sdelete and extract it to your desktop.
3. Form a windows console to run:
   sdelete -c c:/
   This will add zeroes to all the free space in your C: drive
4. To run the following command from linux:
   VboxManage modifyvdi /path/to/your/VM.vdi compact