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Is it safe to delete C:WindowsInstaller on Windows 10?

According to Microsoft employee joscon in the Microsoft Technet post "Can you safely delete files in the %windir%Installer directory?", it is not safe to delete C:windowsinstall or any files within it, and the post explains why as well.

I've referenced and quoted the portions of the post that relate to your question and helped make it more clear and concise for the specific subject matter of your inquiry.

Can you safely delete files in the %windir%Installer directory?

The answer is flatly: "No".

Let's talk about why this is a bad idea:

1. First, it is not supported. If you remove files from this directory
   and have issues, you may need to reinstall the application to get
   back to a good state.



2. 
   

   This particular directory's job is to act as a cache location for Windows installer based applications. It holds stripped down versions
   of the Windows installer data files. During application install,
   update of the application or application removal, this directory is
   used by the application to confirm the existence of previously
   installed items to determine the next steps the installer needs to
   take.

   
   
   
   
   
   • The files are different from machine to machine, so if you
     expect to delete the files in the directory and then copy them over
     from another machine, that would be incorrect. Removing items from
     here could cause you to have application crashes, or worse, require
     the re-installation and patching of the application.
   
   
   
   

The overall idea that you really should not remove items in the
Windows directory. We build and test our software based on the
existence of specific files and directories. When those files and
directories don't exist, bad things can and will happen. However, that
is a generalization that usually upsets many people so let's be more
specific.

The proper way to alleviate space pressure in this directory is to uninstall any unneeded applications.

source

The C:WindowsInstaller folder is where some but not all applications uninstall files and folders are stored.

If you want to remove applications, use the Control Panel Programs and Features to uninstall them. It is also possible to run Disk Cleanup (cleanmgr.exe) in elevated mode to help free up space.

get W10 in a virtual machine and try it out