Fix ERROR_PRODUCT_VERSION 0X00000666 — Another Version Installed

The 30-Second Fix: Check for Hidden Installations

This error means Windows thinks you already have the same program — or a newer version — installed. The quickest check: open Control Panel > Programs > Programs and Features. Look for any entry matching the software you're trying to install. If you see it, uninstall it and try again. No luck? Move on.

Also check the Start Menu for leftover shortcuts — sometimes the program is still running in the background. Kill it with Task Manager if needed. This alone fixes maybe 30% of cases.

The 5-Minute Fix: Uninstall Hidden Copies

If the normal uninstall didn't find anything, Windows might have a broken or hidden version stashed in the Installer cache. Here's how to dig it out:

1. Download Microsoft Program Install and Uninstall Troubleshooter (it's a free tool from Microsoft, still works on Win10 and Win11).
2. Run it. Choose the program causing the error.
3. Let it scan for leftover MSI entries and clean them.

If that doesn't work, try Revo Uninstaller Free — it scans for leftover registry keys and folders after a normal uninstall. Run a forced uninstall on any leftover traces of the product.

msiexec /x {ProductCode} /qn

You can find the ProductCode in the Windows Installer log file (look in %TEMP% for an .msi or .log file created during the failed install). Use that GUID in the command above to force a complete removal.

The 15+ Minute Fix: Registry Cleanup

Still stuck? The installer's registration data is corrupt. You'll need to manually clean the registry. Back up your registry first — export the keys you're about to touch.

1. Open Regedit.
2. Navigate to:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall

HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall

Look for entries matching your program name or publisher. Delete the whole key. Also check:

HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Installer\Products

This one's trickier — entries here are GUIDs. Search for your product name using Edit > Find. Delete any matching GUID keys. Then run msiexec /unregister and msiexec /regserver from an admin command prompt to reset the Windows Installer service.

Reboot. Try the install again. If it still fails, you've got a deeper issue — maybe the MSI file itself is corrupted. Download a fresh copy from the vendor.

Why This Happens

Common triggers: you upgraded a program without fully uninstalling the old version, an antivirus blocked part of the uninstall, or you copied the installer folder from another machine. The error code 0x00000666 means ERROR_PRODUCT_VERSION — Windows sees the product code matches an existing entry.

Skip reinstalling Windows. That's overkill for this. Stick with the steps above and you'll be done in under 20 minutes.