Oh, now I see. Like standard Windows Authentication when you log onto a computer on the network.
I don't think you can mess with this from a web developers standpoint, JavaScript or any other. I think the closest you may come will be client side VBScript which seems to be a little more forgiving on the security. Downside there is that it is only IE compatible.
Search the Knowledge Base at Microsoft and see if you come up with anything.
ToddWW