VirtualBox

1. overwrite empty blocks on the disk with zeroes. See below for applications.
2. run the VBoxManage tool like this: ¹⁾
   

   VBoxManage modifyhd <NameOfVDIorVMHD> compact

[ Homepage ] Install as normal. Then go to Edit → Preferences → Erasing… and select the second Tab Unused disk space. Create a new method with one pass and a pattern of only zeroes. Now go to the “On Demand”-Tasks and create a new one with your local harddisks as target. Run it to perform erasing.

[ Homepage ] Download and unpack. Run it like:

sdelete -c C:\

Use something like:

dd if=/dev/zero of=/junk
sync
rm /junk

This will create a large file with only zeroes and fill up the empty space on your hd.

[ Homepage ] Install the zerofree package. Run it as the man page says.

Seems like zerofree only supports unmounted filesystems. For this to work you should boot into a recovery Linux inside your VM to clean the disk image.

Install the secure-delete package. Run something like:

sfill -z -l -l /

If you just installed VirtualBox 3.0 and noticed that you only see parts of Windows' titlebars and the icons but no text or backgrounds, try the following:

Run VirtualBox like this: Found here

env XLIB_SKIP_ARGB_VISUALS=1 VirtualBox