In all honesty, this isn't new, just another variant of the same theme, social engineering.
All the warnings in the world on stuff like this won't prevent the unwary or those not knowledgeable about their own system will be caught time and again. All things have to be taken with a degree of suspicion and a large dose of common sense.
How can something know your system is infected or is running a scan if you didn't actually download it and give it permission to run, simple it can't and common sense should tell you that. After dashing round the room like a headless chicken for a while, don't click anything and I do mean anything, use task manager to end all browser processes.
Now clear your temp files with something like ccleaner - then you can start to thing about precautionary scans avast, mbam, etc.
Unfortunately, those people reading these security warnings, etc. are the least likely to have been tricked in the first place.
