Avast is stable on Windows 8 or 8.1 and has been running on my Asus sonicmaster s500c since the day I got it back in 2013... I would use defender myself but ime Avast is better for people without any computer skills. link = Avast | Download Free Antivirus for PC, Mac & Android
I concur with Norway; I like Avast and I've got it running on about 150 customer computers. The majority are free version users, and I've got a 99% success rate with it combined with Malwarebytes as above and some additional secondary protection from TrendMicro such as RootKitBuster and RuBotted which keep your computer from getting rootkits and bootkits. If you're not familiar with these types of infection vectors, they are some of the nastiest viruses around as standard AV's such as Defender, Norton, and Avast often can't detect or remove them.
I prefer Webroot Secure Anywhere, it uses behavior, cloud analytics and sand boxing to detect and stop malware. It has 100% detection rating, it's extremely fast and has a tiny memory and storage foot print.