It worked OK for me (none of the username/password strangeness you describe). I went to:
http://www.shrapnelgames.com/joe/ac3/ac3_patch3.1.asp
and left-clicked on "Download Now!". You might try it that way if you haven't already. If it still doesn't work, you could try right-clicking on "Download Now!" and choose "Save Link As..." (Netscape Navigator) or "Save Target As..." (Internet Explorer).