Hi, Guessing I should (per request) turn this off during install. Should I also have it off while using X2? thanks.

I keep it on when using X2 as long as there are no problems. If I'm getting crashing or dropouts, I'll turn it off. If your DAW is on your everyday computer like mine is, the problem comes when you switch it off for X2 and then forget to turn it back on. Sometimes I'll realize that it's been off for days - not good. 

If you're like me you'll probably be Googling stuff and watching tutorials etc when you're using your DAW. If I had a dedicated DAW machine I'd just disable network connections and security altogether. 

And yes it's a good idea to turn it off during install. 

+1 to what sharke says and is exactly what I do :-)

makes sense, thanks!

I installed X2 with MSE still running and no problems. I did the same thing with X1 when I reinstalled it on my new PC about 3 months ago. Many programs tell you to close everything before installing and on early operating systems that was an issue. With the later multi-tasking Operating Systems and PC's its not usually an issue. Some Antivirus programs or security suites of programs can potentially be an issue, but I haven't run across any since I quit using (and recommending) Norton and McAfee.

MSE is very lightweight, I wouldn't worry about it at all.

The only issue I've had with MSE is when I'm loading a program like Adobe Audition that does a scan at startup for plug-ins etc. I noticed that MSE slowed the load time down...it was even worse with Pro Tools. But as a rule I turn off AV software when using audio programs or installing things.