I've been looking at various server monitoring software. I had Nagios setup with the Oreon front-end, but it's pretty confusing to say the least.

I just want my staff to have something simple to use, which will check the server load, ports, and processes. Something with a central installation, but that can check multiple servers (linux, but windows too if possible)

I've found Nagios (installed..not to my liking at all, unless it's just the front-end that's bad), Zabbix (like Nagios, overkill).

check out intellipool.se it's not free or overly cheap but by far has the best features out there for the price range.

create some inhouse scripts..that should do..

You can find something here:
http://www.hotscripts.com/PHP/Scripts_and_Programs/Server_Management/index.html

use MRTG and ipswitch.

Did you check hybodus? Its a bit costly, but easier to manage ..