is that some of your pages aren't coded cleanly.  
for example, i think the main page is missing 
it's < head > tag, though it does have its 
< /head > tag.  also, i'd remove the center tags 
from the main frameset declaration, as i'm 90% 
sure there isn't supposed to be HTML specified 
*outside* of a body or frameset.

i've seen problems like this many a time before.  
Internet Explorer has the nasty habit of being 
*VERY* forgiving for un-clean HTML, and will 
simply guess at the code it thinks you wanted and 
go ahead and display the page.  other browsers, 
including Netscape and others people on Macs tend 
to use will balk at un-clean HTML and simply 
display the page errors intact.

if you run your pages through an HTML validator, 
you should be able to fix your problems easily.