A1 & Regvac do a super job of cleaning my computer.

But I am interested in Cleaning the free space on my drive as well as file Slack.
I consider this do be an important feature as well as a defense.

   When A1Click deletes a file, it uses a process so that the space left behind is clean.
   It really is not important to clean the free space.  It does not improve performance in any way and you will never see whether it is clean or not.  It is like painting the walls of a subway tunnel.
   It is true that if a crime lab went through your hard drive that they may be able to find deleted files in your "uncleaned" free space, but they probably could also (if their claims are true) find deleted files in "cleaned" free space too.
   I think that those programs are just a scam by software companies.  Realize that software companies have to write new programs to make more money and so if you don't need new programs, they have to write programs that you do not need and then convince you that you need them.  Most reviewers know little about computers or software and so they just parrot the press releases of software companies.
   I would think that defragging a drive would clean the free space.
   If you have a hard drive that you want to completely wipe clean, perform a full format of it.
   If you want me to write a program which cleans the free space, I can.  It would be a very easy program to write and I might as well be making money off of it.  I am quite busy for the next few months, but after that, I may just do that.

Ray,
Thanks for your informative response. I was under the impression that when malware
was deleted that residuals can be left behind. Therefore it is a good practice to clean these areas after removal of malware (rootkits,trojans etc.). However, your response indicates that may be software hype. I am sure you know more about this than anyone else I know. Thanks for setting me straight.

   What you say ("when malware
was deleted that residuals can be left behind") is true but is not true.  Yes, when malware or any other file is deleted, it leaves behind remnants on the drive, but there is no threat of malware piecing itself back together and reappearing on your computer.  Let me explain.
   If you were to look at the actual data on a drive, it would be basically made up of one area that is a list of all of the names of the files and folders on the drive (INDEX) and another area of the actual file content (STORE).  The INDEX points to where the file is located in the STORE.  The file usually is scattered across the STORE in smaller packets.  So when you look at a drive with Windows Explorer or in an Open dialog, it shows the INDEX.  When you open the file shown in the INDEX, it sends you to the actual file packets in the STORE and assembles them to make the file.
   The thing is that when you delete a file, in an effort to save time and resources, only the entry in the INDEX is removed.  It does not delete the content of the file in the STORE.  This is not a problem because if it is not in the INDEX, it cannot be used.  Plus, in time the file packets get overwritten with packets from other files.
   The free space in the STORE is like a hobo stew.  Without you having a chart of what each molecule in the hobo stew is and where it is located (as the INDEX does), there is no way that the chicken soup which you poured in it will recollect itself and take over the hobo stew.
   You can get an undelete program which will look at the free space in the STORE and try to find whole files.  If it finds content that has not been overwritten by other files, you can use the program to reassemble it and put an entry back in the INDEX for you.  But that is something you have to do, there is no way that viruses can resurrect themselves from this area.
   The only way that having dirty free space is a security risk, is that if someone has access to your computer and has an undelete program, they can find files that you have deleted.  If you are worried that someone will do this and find incriminating evidence against you in your deleted files, then you need to clean your free space; otherwise, you don't.

Ray,
Thanks for your explanation.
I can see that I was under a false impression.
I probably never have anything on my computer that needs hiding, so your points are well taken.

I believe what misled me was the observation that malware that tried to disable defenses
such as AntiVirus updating seemed to attack and disable slack cleaning Software as a priority. I assumed that was to ensure removal efforts would fail.

Anyway, I consider the question answered. Thanks for your quick response.