if you really wanna wipe data, keep it on a seperate partition. when you are finished, wipe the partition by booting from a linux CD and using this command:
dd if=/dev/zero of=/dev/hda2
where hda2 is the partition.
This will write zeros to the partition.
There's still a small chance of some data being recoverable because some sectors may have been marked bad to write to, but can still be read from. But that's fairly unlikely that there's anything readable there.