Browsing the web recently the above screen popped up.
It appeared that my c: and d: drives were riddled with trojans and security issues. Luckily for me, Windows Defender was springing to my rescue! The eagle-eyed amongst you will already have spotted why Windows Defender was unlikely to be all that useful in this case..
(aside from the fact that the site is a scam aimed at installing a particularly nasty trojan, I was running Linux at the time..)