First, this is not a problem with the client machines so please don't go there. The environment is a MS Server 2003 Network behind a FortiNet router/firewall appliance. Client machines are mostly laptops. When the some (not all) laptops are connected to the company network some websites will not load in *any* browser (IE, Mozilla, or Firefox). MS related sites will not load (Hotmail, MSN, Windows updates). There is no error it will try to load the page for several minutes then just stops trying. The affected machines have nothing in common, they are different models and different ages. However, not all machines are affected. I've taken the IP from a machine that works and put in on a machine that does not, but that did not resolve. I've wiped and reloaded a machine, but that did not resolve. It has affected machines taken out of the box from the manufacturer. And they can still access some pages, for instance, Google pages always load perfectly.
Now, when these laptop (all owned by the company) are taken home by the employees and connected to the employees home networks they can access all pages perfectely.