If you were selling the drive or the entire system you would want something that would guaranty a total wipe of all data. Then you would be looking for a drive eraser like Eraser or Active Killdisk. To simply wipe all files off for a reinstallation of Windows you can use the XP or Vista installer, download the free Linux drive tool known as GParted live for cd, or any other drive partitioning software like Partition Magic(retail) for this.