* prerequisite: the virtual disk image is of type "dynamically expanding" * delete and install in the guest as much as you want * defrag your virtual disk, if necessary (Windows guests) * with the guest running, fill the free space on your virtual disk with zeros. - For Windows guests, use [sdelete](http://technet.microsoft.com/en-us/sysinternals/bb897443.aspx/): sdelete -c c: - For Linux and BSD guests: cat /dev/zero > zero.fill;sync;sleep 1;sync;rm -f zero.fill * finally, with the guest shut down, issue the following command in your host system: VBoxManage modifyvdi virtualdiskname.vdi compact YMMV, but I managed to save 2 GB in this way for a Windows XP guest. **Update:** in VirtualBox 3.\* and later, use the following command: VBoxManage modifyhd --compact virtualdiskname.vdi Furthermore, in more recent versions of sdelete, the command-line options 'c' and 'z' has been exchanged. To zero a disk, you now have to use sdelete -z c: