SDelete - nulling end of drive

To compact your virtual drive you have to:
1) defragment the guest operating system (usually WinXP defragmenter)
2) nulling the end of drive with sdelete:
http://download.sysinternals.com/Files/SDelete.zip
3) compacting the vdi drive from host

VBoxManage modifyhd --compact  vdi_name.vdi

Nulling end of drive (writing zeros at the end) enables VBoxManage to compact the drive (cut out the nulled part) and make the vdi file smaller.