"Turn on Virus Protection" Notification appears after the latest update

Windows Defender “turned off” notification on every boot — fix

If Microsoft Defender has recently started showing a notification at every Windows startup saying that antivirus/Defender is turned off, despite Defender eventually showing as enabled, a recent Defender platform update may be responsible.

In my case, the problem started after a recent Windows update.

My Defender platform versions were:

4.18.26080.4 — problem occurred
↓
4.18.26080.3 — problem still occurred
↓
4.18.25080.5 — problem completely disappeared

I rolled the Defender platform back using PowerShell as Administrator:

$mp = Get-ChildItem "$env:ProgramData\Microsoft\Windows Defender\Platform" -Directory | Sort-Object Name -Descending | Select-Object -First 1 & "$($mp.FullName)\MpCmdRun.exe" -RevertPlatform

Running the rollback again took me from 4.18.26080.3 → 4.18.25080.5.

After rebooting on 4.18.25080.5, the annoying startup notification was completely gone.

If you're experiencing the same issue, check your Defender platform version with:

(Get-MpComputerStatus).AMProductVersion

Important: this is a workaround for the specific startup notification issue I experienced, not a recommendation to permanently disable or leave Defender outdated. A later Microsoft Defender platform update may contain a fix.

Hopefully this saves someone else the headache of thinking their antivirus is actually being disabled every time Windows boots.