Copy all the space to a new file
dd if=/dev/zero of=/tmp/empty.dd bs=1048576
delete the empty file
rm /tmp/empty.dd
Shutdown the server and run the following command in DOS to get in the right working folder
cd "C:\Program Files\Oracle\VirtualBox"
Then run this command to rebuild the disk image
VBoxManage modifymedium --compact c:\xxxx.vdi