404 error when dowloading DLL files from IIS 6.0?

Windows 2003 Server Standard SP1
IIS 6.0
ASP.NET 1.1

My ASP.NET page uses a client-side .NET control. The control is
implemented as a .NET assembly which is located in the virtual
directory root. This worked on IIS 5.0/Win 2000 server, bit IIS 6.0
refuses to serve the DLL file to the client (404 file not found
error).

I saw multiple posts that say that IIS 6.0 does not allow downloading
files of unknown MIME types. I followed the steps outlined here:

http://support.microsoft.com/kb/326965

, but, to my surprise, the ".dll" extension was already set up (as
"application/x-msdownload".

I also added ".*" as "application/octet-stream", but, still, it did
not help.

Here is the entry from IIS log file:

2008-01-22 18:54:37 W3SVC1 172.19.12.33 GET
/oms/Helper.Win.Controls.dll - 80 - 173.29.123.19
Mozilla/4.0+(compatible;+MSIE+7.0;+Windows+NT+5.1;+InfoPath. 1;+.NET+CLR+1.1.4322)
404 2 1260

Any ideas?