Well, in theory, the only way to completely erase FOREVER and ever information from your harddrive...would be to remove everything, and then refill it with useless junk. Otherwise the information remains in the buffer.
Now, if you arent actually running from the CIA...
Reformat. You'll need an operating system disk (a la Vista, XP, or even Linux). If you know your way around a computer, then just pop it in, restart, and do as prompted with the boot disc.
For safety's sake, however, it might be wise to Google some instructions on how to reformat a computer.
I took the liberty of finding a tutorial for you, but it's set up for Windows XP. If you are reformatting with any other operating system, you will notice differences. Alternately, you could probably find a tutorial for any different operating system.
http://helpdesk.its.uiowa.edu/window...s/reformat.htm
Best of luck to you
KFJ