In response to the following post I found on the net…
IE suddenly stops being able to connect to URL where plone server lives, even using localhost, IP and port#’s. (win32 2.0.5 stable, on XP sp2 with all the latest everythings, running a LAN cable connection)
Netscape and Firefox see the server just fine. The connection fails (timeout) or returns an http 500 error.
Google search on this turns up a LOT of similar incidents in about every server config you can imagine, even MS servers, and as many different solutions too.
Here is the one that worked, for me.
Solution: (in IE)
Tools >> Internet Options >> connections tab >> LAN settings >> uncheck the auto detect featureYou likely need to restart the browser, I did a full reboot to be certain.
If you’re like me and always cross browser testing (Firefox, IE, Chrome) and also doing other work, if your network has proxies on like here at my work. In order to view public sites I need to flick Auto Detect back on… this gets very annoying after a few times. So best way to resolve this, is to just keep Auto Detect on and anytime you need localhost, simlpy type in http://127.0.0.1/ - and you’ll get localhost 
Tags: chrome, firefox, google, internet explorer, internet options, localhost