This will 'nuke' everything (operating system, files folders ... the lot) - is that more drastic than you intended?http://www.dban.org/
If you want something less drastic, have a look at Eraser. This description fromTechSupportAlert:
"Eraser will securely erase selected files and folders, or even whole disks, located on your hard drive (it works with any drive, including IDE, SCSI and RAID, and CD-RWs). Eraser can overwrite the data area with your choice of a variety of random data patterns, and can also wipe data in the paging file, Internet cache, temporary files, Internet cookies, unused disk space, and a number of other places where data can secretly lurk. It can handle FAT16, FAT32, and NTFS partitions. Erasing files with a high level of security will always be a difficult and time consuming task, and absolute 100% safety cannot be guaranteed. However, Eraser makes the task about as easy as it can be, at a security level that exceeds most conceivable requirements."
Edited by 1002 Richard S, 03 August 2009 - 10:20 AM.