Start Control Panel, select Administrative Tools, then Computer Management, and then Disk Management. Find your 320 there, and right click on it and choose Delete Volume. Follow the directions, and you're done removing the partition. If you have more than one partition on your 320, you'll need to do the Delete Volume for each one.
Now you can secure erase your 320 in Tool Box. But you should prepare for one last roadblock. You likely will find that when you start the secure erase, you'll see a message about the 320 being locked, and you need to remove power from it. So you might want to have the side of the case off so you can remove the SATA power plug from the 320, and then put it back to release the lock. You may think all of this is difficult, but the other SE methods using a boot disk are more of a pain.