Since you are a Windows shop I would recommend SolarWinds. Orion might be a little overkill for you and it is a little expensive but it is a great tool.
http://www.solarwinds.com/products/orion/
You might also take a look at spiceworks. I am starting to hear a good buzz about them again. Great tool and very simple to manage.
http://www.spiceworks.com/
If you dont mind working on a linux server then my number one recommendation will be Zenoss. Great tool with a solid user and support following. 95% of the config is done via a web GUI. Zenoss has a small learning curve but it can be a very powerful NMS.
http://www.zenoss.com/