Windows.old is for previous versions of Windows for when major updates (like Windows 10 anniversary edition) are released, and it is so that you can easily roll back should there be any issues with the new version. As already mentioned in the above, it should remove the files contained automatically after a few weeks, but if you really want to get rid of it then don't mess around with taking ownership and setting permissions as Windows has a built in option to remove it, which DalekSec has mentioned above. Just use the Disk Cleanup utility to remove old Windows installations and it will do the rest for you.

Well, I'm just wondering. why doesn't anyone recommend Disk Cleanup up first??
It was much simple and easy solution for TS than have to make a Live USB...

this is how you remove that folder.
I can not believe that you are the first to have given the correct solution, really it was so easy.

read the DalekSec message.
Anyway if you don't delete it for some times windows will delete that folder for you. it is just a temp folder if you want to reinstall the previous system.

Because it was already suggested in the other thread the OP linked at, saying nothing there worked.

Ah, I see. Interesting, for I never found a case where Disk Cleanup failed to delete Windows.old folder (up until now obviously :D)
Anyway, thanks for the answer!

But that page doesn't actually mention using the advanced options to delete old OS installations. It mentions using the Disk Cleanup tool, but no explicit instructions on which option to use. There is also a link to another site that explains how, but that could possibly have been missed what with it being on a different website.

I just now saw this. My apologies for being unclear. I didn't try Disk Cleanup to delete the Windows.old file. What I was talking about nothing working was nothing was recovering the 20GB of "lost space" that I discussed in that thread.

Again, my apologies for being unclear on what didn't work. I still haven't figured out why I went from 100GB of free space to suddenly down to 80, but what I did do was delete some "bloatware" that came with the laptop (ASUS Republic of Gamers Laptop) and it got me back to 90GB of free space.

The laptop has a 1 TB hard drive too, which has plenty of space. The 250GB C drive is an SSD, whereas the 1TB D drive is a standard spinning hard drive. I have been installing only newer games that might benefit from the faster speeds onto the SSD (C Drive) thus the inability to understand why it went from 100 free GB to 80 because I hadn't installed anything on there is quite some time. But I just can't figure out what the problem is, if, in fact, there even is a problem.

Again, my apologies for unclear on what I was talking about when I said nothing worked. I meant nothing was able to explain and/or recover the 20GB of free space on my C drive. So I started this thread hoping to delete the WIndows.old file to see if that might do it. And no go. I deleted it and it made almost no difference on the amount of free space on my C drive.

Have you checked the temp directory? That's where applications are supposed to write temporary files to. Usually programs are expected to clean up after themselves but some programmers are lazy, sometimes programs also crash, etc. and over time that directory becomes full of crap that you can safely delete.

If you don't want to delete stuff manually I recommend you use the free version of CCLeaner. That's a very handy program that can clean up behind the system as well as many known programs to free up disc space and protect you privacy.

In that case, you could use programs like or [url=https://windirstat.net/]WinDirStat to analyze which file or directory that eating your free space, hope this help :)