I had the same problem. Through the Avast! Pro Antivirus app on my windows 8.1, after finished a scan, I got a message to scan grime, I checked it out and told me to buy the Grime Fighter to make my system performance better. And so I did, as I assumed since the message came through an app running on Windows 8.1 itself, what it would be advertising would also be compatible. Anyways besides being disappointed for the "scam", I am also disappointed that I cannot use the Grime Fighter as it seemed quite a good tool.
I just contacted Avast! for a refund, hope I get one, if not I'll have second thoughts about upgrading to Premium when the subscription expires in 60days.