If you need to remove/over-write or (secure erase) data on a Hard Disk Drive while you are using Windows, have a look at the clean all #7 option of this tutorial at the link below.
If you want to over-write a HDD before you start an installation of Windows have a look at this one and be sure to post back with any further questions you may have and to keep us informed.
Please post a maximized snip of Windows disk management with a complete list of what is on what partition.
Note
In the Windows start menu right click computer and click manage, in the left pane of the "Computer Management" window that opens click disk management and post a maximized snip of that.