People are very surprised when I tell them that deleting a file in no way removes the files information. All that actually happens when you delete a file is that the file name is set aside and the space the file previously occupied it is marked as ready for new information to be written to. Yes, in theory if enough new information were written onto that file sector, at some point it would be impossible for a data recovery program to reconstruct your original information. But this would be a random process and would be impossible to monitor.
To permanently erase files from your computer you will need a file shredder program. And when preparing to do a complete hard drive wipe Windows users must make sure to purchase a file shredder program specifically designed for their OS and deemed capable of large scale file erasure. The most recommended file shredder programs are those designed and sold by Remo software. As with all Remo products, they are available in a trial download so that you can be sure the program will do all you need of it before you have to pay for it.