If you copy data from your hard drive to another disk, CD or whatever, then delete the original data you copied from, it still remains at it's original location (on the hard disk) even after you've emptied the recycle bin. Windows simply 'hides' it from any file explorer listing and marks that area of the disk as 'available for new data'. But until that area is actually overwritten with new data, files can be recovered from it with any of several 'undelete' or 'file recovery' utilities. And even if that area is overwritten once or twice, some files can still be recovered partially.
It is generally considered that you need to overwrite 'free space' on the hard disk at least 35 times in succession to render the original data non-recoverable by all except forensic utilities as used by law enforcement agencies, but generally speaking over-writing 3 times is sufficient to frustrate attempts to recover old data by normal undelete methods. Even overwriting once is better than none at all, but parts of files could still be recovered with software.
If you want to overwrite your free space immediately for security reasons instead of waiting for several overwrites to occur during the normal course of PC usage (and not knowing which files are actually being overwritten in the process), you can download a free utility called 'Eraser' from here:http://www.heidi.ie/eraser/
Use it either to erase (overwrite) files a preset number of times (instead of just deleting them), or use it to erase (overwrite) all the free space on the disk after checking that the Recycle bin is empty.
Edited by pip22, 06 December 2007 - 12:59 PM.