Microsoft's January 2026 Patch Tuesday security update (KB5074109) is causing serious boot failures on some Windows 11 PCs, often showing an "UNMOUNTABLE_BOOT_VOLUME" error and preventing systems from loading normally.  Because emergency fixes released so far haven’t resolved the issue, affected users must uninstall the problematic update using the Windows Recovery Environment (WinRE).  This involves triggering WinRE (e.g., by repeated power cycles), selecting Troubleshoot > Advanced Options > Uninstall Updates, and choosing to remove the latest quality update.  If WinRE isn’t accessible, you can boot from Windows 11 USB media and use the Repair my PC option to enter recovery mode and remove the update.  Once uninstalled, the PC should boot normally again, and you can pause updates in Settings to prevent immediate reinstallation. 

Quick Guide: Fixing Boot Issues After the January 2026 Windows 11 Update

1. Force Windows Recovery Environment (WinRE):

   • Power on your PC.

   • When the Windows logo appears, hold the power button to force a shutdown.

   • Repeat this cycle 2–3 times until WinRE starts automatically. 

2. Uninstall the Problematic Update:

   • In WinRE, go to Troubleshoot -> Advanced options -> Uninstall Updates. 

   • Choose Uninstall latest quality update (this removes KB5074109). 

   • If prompted, sign in with an administrator account. 

3. Alternative via USB Recovery:

   • Create a Windows 11 USB installer and boot from it.

   • Choose Repair my PC -> Troubleshoot -> Uninstall Updates. 

4. After Successful Boot:

   • Once Windows starts normally again, open Settings > Windows Update and pause updates to avoid reinstalling the faulty patch immediately. 

If you have any trouble with these steps, let me know your specific error and I can tailor instructions!