From my understanding, I had a friend that had avast on his PC. It was slowing down A LOT, so I ran a malware scan, and detected over 2000+ objects. We're pretty sure the source was avast, but we're guaranteed it was.
MSE + malwarebytes has been working great for me
I thought it always did much more harm than good to have 2 different anti-virus programs running on your computer though...I just use MSE, haven't had a virus in like 4-5 years cause im not dumb.
It's not good, get Norton.