How can I be sure the files cannot be retrieved?

The easiest way to do this is to click on the Options button from CleanUp!'s main window. Then make sure the "Fully Erase Files (Wipe Clean)" checkbox is checked. Click OK, then run CleanUp! as normal. When you run CleanUp!, instead of simply deleting files no longer needed, it will first overwrite the files to be deleted many times, rename them many times, then delete them - making them unrecoverable!  :-)  This will take a little longer but it helps protect your privacy even further.

Tip: After running CleanUp! with this option, restart Windows when prompted. Then, run a Disk Defragmenter utility. Windows 95, 98, ME, 2000 and XP users normally have one available by clicking on Start, Programs, Accessories, System Tools, Disk Defragmenter. Windows NT users, unfortunately, will need a third party utility.

More information...

The "Wipe Clean" option provides a "reasonably secure" means of file deletion. It implements a method that used to be the one recommended by the US DoD a few years ago. This involves overwriting the data in a file multiple times with various patterns.

The latest DoD Guidelines suggest that overwriting - even just once -  is sufficient for "clearing" the data, however that is no longer considered acceptable for "sanitization". For that they now recommend "degaussing (with an NSA approved degausser) or physical destruction". For more information, see:

In addition to the multiple overwrites, CleanUp! also renames the file numerous times and changes the size of it multiple times on disk. This is sufficient to deter even moderately technical hackers trying to recover data and will, more than likely, also make it impossible for advanced techniques to retrieve much, if any meaningful data.