I was working on a site today and wondered why a set of links to outside sites ended up pointing to my own box. Ended up these sites were flagged as advertising sites in my HOSTS file. I temporarily commented out the settings but of course this didn't make a difference to the browser. After a quick Google search I found that you can flush the DNS in Windows by simply doing:
Gotta love google! By the way, Netscape seems to keep it's own cache a bit longer. I had to test in IE to see the change.