0x80070660: A Microsoft-Style Error

While updating Visual Studio 2026 with its first minor patch (18.1), the installer threw:

Windows Installer service is unavailable. Please restart your system and try again.

After a quick search, I found many possible solutions, but none of them worked, e.g., restarting the Windows Installer service, re-registering the Windows Installer DLLs, or SFC (System File Checker) scan.

What made it even weirder was that SSMS (SQL Server Management Studio) and many other Microsoft apps also failed to install/uninstall with the exact same error message.

After more digging, I found a more detailed error code in the log files:

The Temp folder is on a drive that is full or is inaccessible. (0x80070660)

That didn’t really make sense either: my temp folder was on C: and C: had plenty of free space. But at least the error code was something I could search for.

Eventually, I stumbled upon the answer from an anonymous hero in some Microsoft community thread:

Create a new folder named “Installer” in the C:\Windows\ directory.

It worked instantly.

I still don’t know why Windows Installer decided to throw this error (it apparently expects C:\Windows\Installer to exist), and why creating that folder fixed it.

EOF