Having two anti-virus scanners running at the same time can cause conflicts.
Note, it is only if they are 'running' at the same time. Most AV's will run in the background, and check every file that you intend to open, so two AV's would fight for the right to access the files.
In the past it was generally easy to 'turn off' an anti-virus, but I am not sure that is the case now since they mostly install processes and services that work continuously, so I would not even consider installing more than one, and attempting to turn off one of them.
So, if you are happy with the AV that you have, then there will be no need to change, in my view.
I have been led to believe that you should not have more than one anti-virus. MS has come out with "Security Essentials" which the information states that it includes anti-virus protection. Can I install this for additional protection or not?