Windows.old folder

 I installed Windows 10 on this computer from the scratch and downloaded & installed Windows 10 Fall Creator Update.
When I check the root directory of C drive, I see not only C:\Windows folder, but also C:\Windows.old folder which is about 30GB.

 Anyway is it ok to erase this folder thru Disk Cleanup/Clean up system files and check "Previous Windows installation ... 30GB" provided I won't go back to prior to Fall creator update time?
 I assume that it was Windows Fall Creator update that had to create Windows.old folder?
 I thought that Windows.old folder is usually created when upgrading Win7/Win8 to Win10 and it is for the case if the user wants to go back to the previous OS.

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Cliff GaliherCommented:
windows.old is created whenever windows is installed onto a hard drive that already had windows on it.  Whether that is from 7 or 8 to 10, or from 10-1511 to 10-1709 (as an example.)

If the hard drive was pristine, no partitions or a partition with no data then no windows.old would be created.  Its existence at least implies that when you installed "from scratch" that there was already a copy of windows on the hard drive.  The other possibility is that the media you used was not the fall creators update (1709) and was older, and then windows update installed 1709 and thus created a windows.old folder. I'd have to lab that though, as I thought updates between feature releases of win10 stopped using windows.old and instead used a hidden folder that got cleaned up after a few weeks.

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sgleeAuthor Commented:
"windows.old is created whenever windows is installed onto a hard drive that already had windows on it." --> That is exactly what I thought.
This laptop came with Windows 8 Pro OS originally on COA sticker. To speed things up, I bought a new SSD,  installed it and loaded Windows 10 Pro from the scratch (was activated automatically), so there should not have been any trace of previous OS whatsoever.
What I am not 100% sure is if I let the laptop automatically apply windows updates (including 1709) or manually downloaded ISO from the Fall Creator Update page and ran Setup.exe ... if I have to bet, I would bet on later theory.
Adam BrownSr Solutions ArchitectCommented:
Some of the updates to Windows 10 create a Windows.old directory upon installation to allow you to roll back to a previous version if something goes wrong after the update. Deleting windows.old will remove that capability.
Jose Gabriel Ortega CastroCEO Faru Bonon IT - EE Solution ExpertCommented:
Yeah. Microsoft has messed up with that update that is another window installation like for too long and it doesn't update their "ISO" file instead of making us download another whole OS to install the new and latest versions of Windows10.

It's quite easy to clean up, I personally use CCLEANER. but windows has the built-in cleaner:

in there you got an option to clean up "old windows installations".
That would clean up a lot of files on the windows.old folder.
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