As an afterthought, it is probably not easy to do. However, you may find a utility that will Secure Delete (overwrite a few times) your blank space on the Hard Drive. When you put items in the Trash can and delete them, the area that they used to occupy is marked as being writable again. Whether anything actually gets written there depends on the amount of stuff you install or move around. The OS will try to keep all files and programs in a contiguous area but it will also put parts of files and programs in places that it sees as being free. Secure Delete will mark the area for writing and then Write a series of Random 0's & 1's to that area. I believe it does this a number of times which is why Secure Delete takes longer than the usual delete.
Try this utility for size: http://www.apple.com/downloads/macosx/system_disk_utilities/shreditx.html
hope this helps
According to Apple, when you empty the trash on your mac, bits the files still stay on your computer and can later be recovered. To permanently delete the files you must empty 'secure' trash. I only found this out recently and the majority of my files have been deleted in the normal way. Some of these files may be confidential and i was wondering whether there would be any way of making sure these files were permanently deleted from my mac?
Thanks for any help!