We (The company where I work) have learned that there are issues with the 2-10-2004 Critical update patch from Microsoft. If you install it on an Exchange 2000 server, it interferes with the web mail interface. Specifically, clicking on a message results in a 404 error if you're using Internet Explorer as your web browser.
Netscape and Mozilla seem to work fine, so the bug seems to be in the code Microsoft uses to add functions to IE users that are not available in other browsers.
This happens when the patch is installed on the server, and it doesn't seem to matter if the client has the patch installed.