If you want to cleanly wipe your NTFS hard drive system, so that nothing can be retrieved from your system, then you need to format your drive a couple of times. To be precise, you need to format the drive 7 times, to be sure that nothing is left on your hard drive.
Go to either www.download.com or www.tucows.com and download ‘Hard drive erasers’, using which you can erase all the data out of your system.
A better option is to format your system completely and reinstall the Operating System (like windows or linux). If you have windows on your system, make sure you have a valid Windows CD, with a valid serial before you format the system. Most of the times you save your serial key on your system in a file where you remember, so write it down on a piece of paper before you format your system.
Once you format your system :
1> Put the valid Windows Cd in the CD-Rom and boot your machine. Most of the Operating System CD’s are self-bootable CDs (they boot by themselves using the auto run feature), but in case if it doesn’t start up, then you’ll have to make changes in BIOS to set your CD-Drive as the first boot-able drive and make your hard drive the second disc to be checked.
2> Once the CD boots, everything is self explanatory. Just follow the instructions and reinstall the operating system.