My suggestion would be either one of two things:
First, the utility you can download from the manufacturer of the drives does a nice job of this and the download is free. It is also fairly quick in comparison to a complete cleaning.
Another great solution (takes more time) is Dban which will wipe EVERYTHING off that disk. Just be sure if you use Dban (it wipes everything and I mean everything) that you do not have any drives hooked up that you want to keep the data on, because it has the ability to just clean everything out in one operation.
You don't want to just format, you want to Dban that disk.
To completely erase all data on the drive(s), download DBAN
and create a floppy disk to use.
Enter the BIOS setup menu and set the boot order to 'floppy first', then reboot with the DBAN floppy inserted in the drive.
It will completely and securely wipe everything off your hard drive. It will then be clean and secure so the next person can be ready to load the new Windows OS on the disk and none of your data will be there.