Hi,
We have neotech als ourer default mail client but it think it sucks. I want to have installed horde is this possible? I also want to have it as the default mail client so when you go to site.com/webmail he start with horde.
You can install in in /var/www/html/webmail overwriting the default or you can install it in a different location and modify the /webmail Alias in Apache's httpd.conf pointing to Horde's installation.
Installing it is one thing, but you have to patch it regularely and first of all configure it to meet your needs since there a lot of plugins available.
You should seek a server admin that knows Horde.