This is a limitation of having IE 6 installed with this tool. Even though it is not your default browser it is the engine that it will try to use. One way that I ensure that I can test for IE 6 compatibility is by making use of IE tester. It is not exactly the same as a native IE6 install but it works very well.
I left IE6 installed on my xp machine for quite some time just to have the browser around to test with, but I gave it up recently as it seems industry wide support for IE 6 is dropping off.