Installer C:\Windows\Installer

Windows Installer cache: copies of the .msi and .msp files needed for repair, update and uninstall.

Updated on 12 November 2026 By AltF4 Formation

What it’s for

C:\Windows\Installer is the Windows Installer cache: it keeps the files Windows needs to repair, update or uninstall a product installed with MSI. Without it, those operations ask for the original media.

What it contains

A copy of every installed .msi package, under a random name such as 1a2b3c.msi.
Patch .msp files: Microsoft states they are cached here.
{GUID} subfolders with product icons, and $PatchCache$ , which keeps baseline files so patches can be uninstalled.

The random name tells you nothing about the product. The registry links them: each product’s LocalPackage value points to its .msi (see the command below).

Risks and best practices

Delete nothing by hand: Microsoft advises against it. Without its .msi, a product can only be repaired, updated or uninstalled with the original media, if it still exists.
An .msp should only go away by uninstalling the patch from the product it applies to.
To save space, uninstall software you no longer use. Avoid “cleanup” tools that remove files they consider orphaned: they get it wrong, and a lost file cannot be rebuilt.

Useful commands

# Size by extension (read-only)
Get-ChildItem "$env:windir\Installer" -Force -File |
    Group-Object Extension |
    Select-Object Name, Count, @{n='MB';e={[math]::Round(($_.Group | Measure-Object Length -Sum).Sum / 1MB,1)}}

# Product -> cached .msi (LocalPackage)
Get-ItemProperty 'HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData\S-1-5-18\Products\*\InstallProperties' |
    Select-Object DisplayName, LocalPackage | Sort-Object DisplayName

Report a mistake