Dynamic site...losing "www." when visiting folder

Not a big deal and I’m sure this is kind of a stupid and noobish question, but nonetheless, i cannot figure out how to keep the “www.” prefix from dropping in the browser URL window when I direct the browser to a folder (containing index.php) rather than directly to an “index.php” file.

This site below, however, seems to manage just fine when you visit alternate sections of the site.

Why is this, and is there anything I can do about it? I didn’t even know what to search for regarding this question.

http://www.mobia.org/programs/

thanks