Can files ever be recovered after doing a 'wipe free space' (or a couple of free space wipes)? Is there 100% certainty that they're gone forever, and no one can ever recover them again?
Usually depends which algorithm / how many times/passes you overwrite/wipe the space/HDD.
In CCleaner it depends how many passes you choose, there's max "Very Complex Overwrite" with 35 passes. Would be very difficult or impossible to recover the data after wiping with 35 passes (though I wouldn't say anything's completely impossible).
In the known reasonable universe there's no way of recovering data from a particular cluster on a modern HD (say under 10 years old) after it has been overwritten just once. Many have tried, and many have failed, and many have posted this on the www. There are claims that a few bytes have been recovered, but you could probably get lucky with a few bytes out of a cluster just by tossing a coin.
This is not quite the same as saying that all deleted data in free space will be unrecoverable after a WFS. Some data will be left in the MFT, some in the paging and hiber files, some left in bad blocks, some left in transaction logs, some overwrites may not replace the original data, and all the other things I can't think about or don't know. This is where the specialists retrieve 'overwritten' data from. Should you worry? I don't know, I don't.
SSD's are quite different, I should add.
Interesting, is it more difficult to recover data after a "very complexe overwrite" (or after more WFS's), as nodles said? I'm going to sell my computer, and I don't want the next owner to recover private documents.
I contend the major security lapse for retrieving sensitve data is the paging file and hiberfil.sys.
You can have Windows overwrite the Paging File at every shutdown with Fixit 50463, and disable Hibernate with the command ►
powercfg -h off
W.F.S. does NOT remove any UNDELETED files on your System Partition (typically C:\)
Windows records far more of your actions than you realise, and wiping only free space will leave a treasure trove for the purchaser.
Windows cannot erase itself and CCleaner will not even try.
You need a Boot CD, such as DBAN NUKE