.exe file to .com file ?

Others may disagree, but I wouldn't rename a .exe file except in extreme circumstances. Like you suspect malware is disguising itself as a .exe file. Changing the name would most likely make the .exe file not work as designed. I especially wouldn't change iexplorer.exe because it is the main executable of the Internet Explorer browser as well as being a critical part of the Windows operating system. Internet Explorer is more than just a browser. It works as an underlying technology behind a number of internal Windows processes including updating, basic Windows functionality and more.

There are methods outlined on some other websites that appear to completely uninstall Internet Explorer and provide workarounds for the problems that removing it causes, but I don't recommend them. In my experience, removing IE causes too many problems to be worth it, even with the workarounds.

Even though removing Internet Explorer isn't a wise option, you most certainly can safely turn it off and use your alternative browser as the one and only way to access the Internet.

Windows Features - Turn On or Off

I don't know why you would want to. Just for grins I tried renaming some small exe programs I wrote to .com. They worked just the same. I guess Windows is smart enough to look inside the file for PE header info. But I only tried a couple of simple programs.

If some program is explicitly launching the application the launch may fail due to filename not found. Maybe that's what you want?