What is this?  I'm confused is it an actual antivirus program?  I purposely don't have any antivirus programs installed on my Sonar computer.  But this downloaded and installed today with the latest windows updates.  What's the verdict on it... leave it or remove it?

I use it. It is not intrusive and it works. 

I use it also, and have for several years now. As John states it is unobtrusive which is a bonus in an AV tool.
 
You mention you don't use an AV app yet apparently you are able to connect to the internet and download files. This is something I would strongly recommend against.
 
Regards,

Yeah everyone tells me the same.  I'm sure I'm due for a major virus and an exploding computer.  In my defense this forum and other audio sites like native instruments are the least-mainstream sites I ever visit.  I'm on a strict diet of facebook, espn, amazon, and yahoo email.  I've worked like this for at least 10+ years and never been bothered.  Obviously I don't open ANYthing via email unless I'm absolutely 100% certain I know exactly what it is.  I'm not saying I'd recommend it to anyone, just that it's possible.

You can turn it off - real time protection and the app itself before making important recordings.
 
While recording it is not likely to have much impact, but when you hit stop, it will scan all the new Wave files unless you do something to stop it. Not a problem if it's the end of the night, but if you are trying to hit stop and start between two songs, you might suddenly find your PC with extra load as you start into the next tune.
 

I don't thing MSE works that good, so I bought the Pro version of Avast. I haven't had any problems since.

That's all I use and the builder of my computer said it should be just fine as long as you don't go to any *unsavory* places.  Audio forums like this should be fine, figuring you don't open any links from those morning posts we sometimes get.  ;-D

I use Security Essentials all of the time.  I have never found it to interfere with Sonar, even during downloads and installs. 
 
Glen

It works well for me.  I started using it under Vista and have had it working ever since.  I have not had a problem from it or due to its not detecting nasty stuff. But I don't go to places that are troublesome anyway.

It works well for me also and is un-intrusive with Sonar. I'd keep it.