The C:\Windows\Installer folder growing to 20 - 50GB in size.

Hello Vincent,

Thank you for posting question on Microsoft Windows Forum!

The C:\Windows\Installer folder which is the directory storing cached Microsoft Installer (.msi) files and patch (.msp) files required for repairing, modifying, updating, or uninstalling software. Because application update loops (especially older Microsoft Office versions) continually pile new patches on top of old baselines, this folder expands relentlessly over time. Microsoft explicitly blocks Disk Cleanup and Storage Sense from touching this folder to prioritize application stability over disk space.

Please be aware: Never manually shift-delete random files from C:\Windows\Installer. Doing so will permanently corrupt the local MSI database, leaving endpoints unable to upgrade, patch, or uninstall the affected applications, resulting in the dreaded Error 1612: The installation source for this product is not available.

The following are some suggested steps for long-term preventative measures.

• Since many large installer caches come from hundreds of small .msp patches. For MSI-based Office, Visual Studio. You can use the Office Deployment Tool to download and integrate the latest updates into a baseline installation source. This way, workstations install a pre-patched version and accumulate far fewer incremental patches.
• A Group Policy can stop the caching of baseline .msi files entirely, preventing future growth. The path is Computer Configuration → Administrative Templates → Windows Components → Windows Installer and the setting is Turn off the caching of Windows Installer files. However, be cautious with this approach. As repairs, modifications, and uninstalls will require the original installation source (network share, DVD, or SCCM). If that source is not available, operations will fail. Only use this in tightly controlled environments where source media is guaranteed to be reachable.
• On the other hand, you can explicitly instruct the Windows Installer engine to stop caching original baseline files when new patches are applied by disabling the Patch Cache via registry with the MaxPatchCacheSize policy. For more information https://learn.microsoft.com/en-us/windows/win32/msi/maxpatchcachesize#:~:text=If%20the%20value%20of%20the,Policies%5CMicrosoft%5CWindows%5CInstaller
• Since querying the Win32_Product WMI class can leave temporary files behind in C:\Windows\Installer, making the folder grow and potentially filling the C: drive. Try to minimize or stop using Win32_Product anywhere in your environment. Scripts, logon scripts, scheduled tasks, monitoring, SCCM/MECM, or third-party inventory tools that call it. For more information https://support.microsoft.com/en-us/topic/temporary-files-are-left-in-the-c-windows-installer-folder-when-you-query-for-the-win32-product-class-on-a-computer-that-is-running-windows-server-2003-windows-vista-or-windows-server-2008-9fb78cee-26dd-8809-45a3-dae95c5e7c37
• If you use WSUS or SCCM, making sure you decline superseded updates regularly (this reduces the client’s SoftwareDistribution folder, not Installer, but helps overall disk pressure) and configure client policies to “Remove expired and superseded updates” automatically.

Hope the above information is helpful!