You do realise those are file packing methods, not file compression methods (the actual containment of the packed files) right?
If you examine the list of compression methods that both products support, 7-Zip comes out on top. 
7Zip and WinZip. I have both of them. WinZip is a paid tool but has more features then 7Zip. Why not?
Eraser and Mil Shield. Again, I have both of them. Mil Shield is a paid tool but has more features then Eraser. Why not?
$29 are nothing as for a such class of programs. Personally, $29 will not kill me. 
Alex, you may want to read this http://www.winzip.com/comp_info.htm
Lot of compression methods, huh?
I second AlexDawson - Use Eraser.
Just out of curiosity - what mode do you use for wiping? On work system I have scheduled task for weekend that runs ā35-pass Gutmanā on free space, but for ocasional erasing, I use - ā1-pass PseudoRandomā.
After erasing free space there is defragmentation schedule using MyDefrag (used to be called JkDefrag) which is free as well.
CCleaner does so.!
Here you are, donāt argue which tool is better. Iāve set both programs on 35-pass method of wiping, see the PNGs :
p.s: The 35-pass method is very secure but the shredding is slower. If the shredding is fast, then the method is 1-pass zero or 1-pass random overwrite.
Why? Paying for a piece of software when a free product does the job better is absurd, youāre just wasting money.
Mil Shield is fine, Alex.Download it, install it, then run it and test it.You will see.
no problems, ma122zda.
Iām now testing them and I have not any experience yet. thanks guys, you are the best forum
I donāt know what āusefulā features Winzip has that 7-Zip doesnātā¦ though 7-Zip supports a MUCH wider range of compression formats
I can confirm that MilShield is very serious software (Iām using it more than 2 years).
It only has 22-23 days free use, but still there is no free and at the same time very good cheese. 
I even think that its price is low (has tons of features). You can have it for free (22 days, I think).
And it wipes (in my opinion and a few friends of mine) the free space satisfactorily without PC troubles after the wiping.
I use Eraser on my work / home computer and itās done the job perfectly fine for me over the last 5 years.
Iāve no idea why anyone would recommend Mil Shield, Eraser does everything Mil Shield doesā¦ and it does it better (for free).
AlexDawson, thanks for this answer. And aranac111, thanks too.
Iāll test both Mil Shield and Eraser wiping tools.
AlexDawson, did you test them ever?
Why is it that not a single person in this thread bothered to read what the original poster wanted? He does NOT want a mechanism to partition or format a hard disk, he wants a method to securely erase the unused (aka FREE) hard disk space on his computer (such as when you securely wipe data from a disk). 
ma122zda, in answer to your question, the tool I recommend is Eraser: http://eraser.heidi.ie/, Itāll wipe individual files, folders or the free disk space.
You donāt really need any special software to do this.
Just go to āMy Computerā, right click the drive you wish to wipe and choose format.
Quick format is likely enough for your purposes, so you should be able to just click āStartā and go. Be sure that you donāt have files you want to keep on that drive first!
Bear in mind: this has to unmount the drive in question, so it canāt be done on drives that have open files. You will have to get the drive into a point where you can safely remove it first.
Hi. You have a few choices. The best one in my opinion is to add another drive as a slave. Copy all the extra files you have cluttering up you old drive to the new one. For those file you really want gone, something like. Hope it will help you.
Good day.
take a deep breathe and relax
i guess i misunderstood it. but i still think most users will do better with two partitions 
and for the unused data, i would use killdisks wipe function.
ma122zda, you probably know that when Windows deletes a file, it does not actually delete the file content. Instead, it just declares the space that was taken by the file as free and each hacker or even normal guy can easily retrieve it with a some recovery utility.
The tool I recommend here, is Mil Shield or Eraser. Eraser is free but I like more the free space shredder tool of Mil Shield (it has all passes and identifies all your external/internal hard drives).
See here:
http://download.cnet.com/Mil-Shield/3000-2144_4-10493437.html
http://download.cnet.com/Eraser/3000-2092_4-10231814.html?tag=mncol
use disktools in win7 to create another partition containing the free space (or as much as you need of the free space), then format it and youāre done + you now have ā2ā disks. one for operating system and another for storage.
no need for another tool