Any user can cause any browser to send any string in the navigator.appName.
Opera is the easiest to set, and early versions sent an IE string by default, if IE was already installed.
You can use object detection to identify IE, but browser support changes constantly, so you have to keep up with any changes.
navigator.isIE= !!(!window.addEventListener && window.attachEvent);
if(navigator.isIE)// do IE only code
else// do non IE