Problem with permanently wiping files and traces (ccleaner)

I have some old files (mostly emails) I want to get rid of so that they are gone for good and not recoverable. These are files that I have deleted in the past non-securely by simply deleting them from windows. As they were deleted the old fashioned way, I ran Ccleaner to wipe free disk space, assuming it would get rid of them. However, when I use software like Recuva, I'm able to find and recover those files even after an MFT free space wipe with Ccleaner.

Recuva shows these files as unrecoverable and I can see that they have been previously overwritten by other files. The files that have overwritten these seem to mostly be newer e-mail files or random windows files, so I assume this is done by windows when the disk space is needed.

Here is the problem though - they show up as unrecoverable in Recuva, but so far I have been able to recover every single one I've tried. I can recover the email messages and even attachments on them show up. so clearly they are not completely overwritten, but I don't understand how/why.

if I try to do a secure overwrite with Recuva on those files, it will not do so because they are "previously overwritten". Yet they are not, because I can still recover them. Ccleaner apparently won't do anything to them, I assume because they are supposedly overwritten already, but again, they seem not to be.

So any ideas on what to do? I wan't to remove all traces of these files. I obviously cannot use a shredder on those files because they are in theory overwritten so I can only see them in recovery software, but still they remain recoverable. Would using another wiper such as Killdisk to wipe free space help? Do I need to nuke the entire drive? Do a clean windows install? or will I still have traces left? replace the hard drive and torch the old one in flames?

please correct me if I'm wrong, but wouldn't all of the above suggestions apply to the following:

a) wiping free space on a drive - which I have done with ccleaner (with these settings: 1 and 3 pass, secure deletion, wipe mft free space, wipe alternate data streams)

b) securely deleting existing files/folders with a shredder tool - which I cannot do, because the files are already deleted and supposedly overwritten by windows

So my problem is that the files are already deleted, so I can't use a secure deletion tool. They are already overwritten so apparently ccleaner or Recuva will not touch them. However, Recuva is still able to recover them so they are not permanently/securely deleted afterall. What to do?

Create a backup image with Macrium Reflect Free using the standard option which only copies current files, not previously deleted ones. Then secure wipe the entire hard drive using a program like MiniTool Partition Wizard. Then boot from Macrium Rescue disc and restore the image. Kyhi's Recovery Drive has both those utilities on it. You'll need an external storage location to hold the disc image, though.