C:\Windows\Installer is 5.7 GB and deleting it breaks uninstalling

Turn on hidden files, open C:\Windows\Installer, and you find a folder of .msi and .msp files with names like 1f4a2b.msi. It is often several gigabytes and it looks exactly like a cache someone forgot to clear. It is not.

What it actually holds

When a program is installed by Windows Installer, a copy of the installer package is cached here. Windows needs it later to uninstall the program, to repair it, and to apply patches. The file names are randomised because the folder is an index rather than something a person is meant to read.

Why deleting it is the classic mistake

• Uninstalling stops working. Programs whose cached package is gone cannot be removed through Settings, and the entry stays in the list forever.
• Repairs fail. Office and Visual Studio in particular lean on this to fix themselves.
• Updates fail. A patch that needs the original package cannot apply without it.

This is the folder behind most of the horror stories about a cleaner breaking a machine, because it looks disposable, it is large, and nothing in Explorer explains it.

How to shrink it properly

1. Uninstall software you do not use. The cached package goes with it, which is the only routine way this folder gets smaller.
2. Remove orphaned packages with a tool that checks each file against the installed programs first. PatchCleaner is the one usually named, and the rule is that it must verify rather than assume.
3. Leave everything else alone. The remainder belongs to software you still have.

Working out what is in there

Get-ChildItem C:\Windows\Installer -Filter *.msi -Force |
  Sort-Object Length -Descending | Select-Object -First 15 Name, @{n='MB';e={[math]::Round($_.Length/1MB)}}

That lists the largest cached packages. The names are meaningless, but the sizes tell you whether this folder is worth any further thought: under a gigabyte and it is not, several gigabytes and it is usually one large suite.

Where the space actually is on most PCs

Installer is a real folder with a real size and it is rarely the largest thing on a drive. Why the C drive is full when nothing looks big covers the order to check, and the component store next door is the one that looks bigger than it is.