obviously if you have files across your zencart installation that has had changes made to your code then you have a bigger issues to worry about.

in any case comments are dependent on the langauge you are writting in.

<!-- --> for instance applies mainly to any html based coding.

example <!--<h1>wow</h1>-->


// comments out a single line in most serverside scripting langauges, in this case PHP which is contained between <?php and ?> tags

example <?php //$wow = 'wow'; ?>

I hope that makes sense as its off the cuff explanation oversimplified but I would suggest having a professional have a look at what exactly is going on with your site files.