"your PC may have been exposed to a virus" unclosable screen

On my wife's laptop this screen keeps apprearing. It cannot be killed with alt F4 and it covers everything so we can't see task manager and we cannot even see the run box after pressing Windows key + r. Starting new desktops doesn't work because they all have this on them. The only way to stop it is to shut down the laptop.

We've run lots of antivirus scans and nothing shows up. Tried clearing all notifications from websites in edge and chrome but it's still happening.

So we're trying to find out a) how to close the window and b) where its coming from.