in vbox
1- Defragment hard drive (in guest system)
2- Write zeros to hard drive blocks of free space (in guest system);
example in XP(guest) run: “sdelete -c” (sdelete is a free utility u can download)
3- Compact the VDI virtual disk image (in host system):
run: “VBoxManage modifyvdi %vdisk_name%.vdi compact”
in vmware:
vmware-vdiskmanager.exe -k %vdisk_name%.vmdk