404 on Windows/IIS with ISAPI_Rewrite simply won’t work

I’m at a loss here. My WordPress blog runs fine on my Windows/IIS server with the standard set of ISAPI_Rewrite rules (Articulate Blog).

The big problem I have now, though, is that my 404 page in my blog simply doesn’t work, even though 404.php exists in my custom theme (under \wp-content\themes\<theme name>). As you can see from the below, it simply returns a page that looks like no search results were found:

https://www.articulate.com/blog/nothinghere

The difference, though, is that it’s really NOT the “Sorry, no posts matched your criteria” page. Compare the above with this:

https://www.articulate.com/blog/?s=theeee&x=0&y=0

This is really important for me to get fixed both because it would be great to have a 404 display when it should (as the rest of my site does), but also because I have a Google Mini, and it’s indexing non-existent pages under my /blog directory since WordPress is not properly returning the 404 page, and thus, a header status code of 404.

So if I can get the actual 404 page to return properly, adding the PHP code to return the proper status code is easy enough:

<?php header("HTTP/1.1 404 Not Found"); ?>

I’ve also posted a comment on a related blog post here.

I’m running Windows Server 2003.

Do I need to add a new rewrite rule to account for 404s?? Make some change in WordPress? I’m leaning toward this being a WP issue since Helicon Tech says the 404 is not specified in ISAPI_Rewrite rules and I have 404 error handling for non-WordPress content specified correctly in IIS, so that leaves WP.

Any suggestions??

Thanks in advance!