Is the store going to take it back with an empty hard drive if you bought it with stuff on the hard drive? Maybe not.
Doesn't Active Kill Disk require operation from a boot disk of some type? That might be over this user's head.
If you REALLY DO want to erase the ENTIRE hard drive, it might be easier to identify your hard drive manufacturer, go to their web site, download their free drive utilities package and run it. They nearly always can write zeros to a drive to eliminate all data.
If you DON"T want to erase the entire drive, I'd guess you will have to restore it to factory retails specs, which would probably be done by a recovery disk or through a recovery partition on the drive.
If by "my data" you mean letters to grandma as opposed to something that might result in identity theft or doing 2 to 14 years in the electric chair, then just navigate to it and delete it.