Does anybody know whether or not this is possible on NTFS? I know it's extremely difficult on ext3 and other journalled filesystems because even if you shred the main file, you still end up with unlinked copies. If NTFS moves stuff around (ie, defrags) without erasing/overwriting it, or if it keeps a history of the file, then you may not be able to delete it securely.
If the data is highly sensitive, you can also consider DBAN, it's a boot cd that'll wipe the drive. We use it in government for certain systems.