Now, in regards to your second question, on how to AVOID having the users log into the web site when they are in the office, the only way I have gotten that to work is to NOT use the IP address of the web server when in the office.
So, if your web server has an IP of 10.1.1.5, and say it has a machine name of MYWEBSERVER.
With the Windows challenge/response enabled, going to http://10.1.1.5 will bring up the login box.
For the users who need to get in but you DON'T want them to log in, have them go to:
http://MYWEBSERVER
That way, they will communicate to the web server over Named Pipes, just like if they opened a shared folder on the web server. The web site should open with no challenge/response necessary, because the integrated authentication is being handled at a server level, as opposed to a HTTP protocol IP address method.
There is probably a better way to describe why it works, but I don't know it! It does work, that is all I know!
hope this helps