http://www.google.com/support/accounts/bin/answer.py?hl=en&answer=54053 (see "CLEARING YOUR BROWSER HISTORY")
I'm not sure if IE6 or IE5 has this feature, but in IE7 under the browsing history (Temporary Internet Files) settings, you have the option to check "for newer versions of stored pages" every time you visit the Web page. Basically, you wouldn't need Temporary Internet Files (aka "the browser cache") anymore.
If a Web page does things correctly (i.e. by setting the "Last-Modified" HTTP header and using "Cache-Control" (HTTP 1.1 only)) and the browser uses the headers correctly, then those settings are useless. For a chatroom, the Last-Modified header should be dynamically updated when the page is reloaded. As a result, the browser checks (or is supposed to check) the page and download anything that is new.
That is how it SHOULD be done. Unfortunately, not all content authors are aware of such things. For that reason, your setting of reloading every time is sometimes a necessity.
In any case, I've given my thoughts on the IE settings you mentioned. Have fun with it. ^_^
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Waiting patiently for Windows 7, XHTML 2.0, CSS 3.0, PHP 6.0, the ratification of C++0x, and the day that I can code without logic troubles.