That's what these shredder programs do, they write over the data, once or more, depending on which option you choose.

Look for a program that gives you the option of high amount of overwrites. I think 30 something is the highest available, that u can buy.

Webroot Window Washer is a good product that offers that high amount of overwrites, and won't put any spyware/adware on your computer.
Free trial.