Some of the free diagnostic tool software packages you can download from HDD makers' website contain a tool for "Zero Fill". This will write all zeroes to every part of a drive. HOWEVER, many of these tools only work on that maker's units. So, for example, if you have a Seagate or Maxtor drive, get the Seatools package from Seagate.
A zero-fill will over-write every part of the drive. Be aware that a sophisticated user with the right tools still could recover a bunch of data from a zero-filled drive. But for >99% of users, this will make all old data impossible to recover. For high data erasure security you need military-quality disk wiping software, and lots of third-party packages have that available. I used to use Norton Utilities years ago that had this.