Sometimes more is not better.
Over the years I have encountered a number of situations where PCs have been running more than one antivirus product concurrently.
While on the face of it this sounds like a reasonable strategy; what one misses the other should get...right?, it really is a fatally flawed strategy.
The flaw is that each antivirus product is designed to know what viruses look and behave like, and to other antivirus products, what antivirus products do IS look like viruses themselves.
So when you install a second or more antivirus product, the first thing they do is go to battle against each other.
Some products detect the problem and refuse to install or, or insist on removing the other product first, but many will just press on regardless and leave you to work out what is going on.
The results of these battles, at best, will be that your computer slows down. In fact, it may well grind to a halt as the battle takes precedent and rages on.
Sometimes these AV battles end up with both sides winning...or losing, depending on your view, and your PC ends up being left vulnerable as none of the AV products is left in a functional (read "protective") state.
A misbehaving antivirus product, or two, can be a very difficult beast to diagnose and somewhat tricky to fix.
If your computer is behaving weirdly and slowly but you haven't seen any real "viral" behaviour then you should probably check what you've got going on Antivirus-wise.
Once these sorts of battles have taken place, and you are left with crippled antivirus software, just Uninstalling or Removing them may not be enough to return the status quo.
Often (read very often) the Uninstallers don't work and leave quite a mess behind.
If you've had more than one antivirus product on your system and you assume that only one of them is misbehaving, you can bet you'll be wrong and waste a lot of time trying to work out what is going on.
Your default and quickest route to sorting the battle out is to use force-removal tools to remove ALL the antivirus products you have and have ever had, on that machine and then add back just one.
Some of you are probably suspicious of the term "force-removal tools" but you needn't be.
All legitimate antivirus vendors provide special tools to fully remove their products for just such incidents.
Never underestimate the weirdness or the persistence of a misbehaving antivirus product.
Over the years they have caused my customers and I great pain. I am sure they will again.
Remember, as your Grandma used to say "one at a time is good fishing!".