One good, quick way without another PC or messing with cmd is to get the Windows installation media and delete all partitions (including all system partitions) then format, when it gets to the point where it lists all attached disks. You won't have an OS at all then though.
Go into the standard Windows drive management window and do a format of the drive.
It should be perfectly fine for re-installation etc.
As previously mentioned though, d-ban is great for being thorough, so probably better if you're selling.