As you may know, when you delete something from your hard drive, you don't really delete it, the space is just marked as available for use. So until you over-write it with some other file, it still sits there on the drive and can be recovered using utilities designed for this purpose.
The good news is that you can use the Windows utility cipher to overwrite the free space on your NTFS drive, thus truly wiping out those deleted files.
From the command line:
Where "C" can be any drive letter you want to wipe the free space on.
More detail here: