Weird problem with Navigation Bar

I have a weird problem with a nav bar, set a navbar on an index.html composed
of Home | About Us | Library | Forum . Now oringinaly the page loads in index
of course, I can enter any page I want, the problem appears for example if I go
from Home to Forum, and want to go back to Library for example I cant. The nav
bar is acting weird. I uploaded the website to www.cain152.3x.ro . Can anyone
help me with this problem? Thanks.