Thank you for the response and I apologize for the delay in getting back. For about a year or so, I would run both the free Avast and free Malware Bytes. Never a conflict, never a problem. They only ran when I opened the program and requested a scan. I freely accepted any upgrades that came along.
I uninstalled Avast completely and figured I would just use the other program, malwarebytes, since Apple recommended it. I took a chance and accepted the trial premium version and activated the real time protection. My MAC slowed to a crawl on start-up. The real time protection, or the entire premium version was eating up the cpu and I would get the spinning beach ball. It took an apple tech to get me straightened out and uninstall the program. My system runs fine now.
It is an early 2009 iMac, running El Capitan 10.11.6. Apple thinks my system might just be getting too old to work with some of the software, or if it works, might slow the system down. And it was suggested that I look at getting a newer iMac within the next year.
So I am hesitant to install any anti-virus software now. I am not tech smart and when the computer had performance problems, I feared the worst. Malwarebytes would complete a scan in a couple of minutes and scan 11,000 plus object and report nothing found. I would then run Avast because I liked the deep scan that Avast performed, 900,000 plus files even though it took over an hour. And it initially found a few items that malwarebytes missed, adware. That was about a 10 months ago. After that, neither program found anything. I do miss the feeling of security, just not sure I want to try again. To be honest, while apple recommended free malwarebytes, I always felt better with running Avast. My system is old. I worry that I will download a version of Avast that will cause my system to run slow.