problem: apache/mod_isapi cgi returns html source code to browser

Hello !

We have an Apache-server with the mod_isapi extension. We use
mod_isapi for an cgi-script (a third party binary dll). Every time we
submit a form and post the data to the dll the browser displays the
html source code of the result page. The source code starts with
"Content-type: text/html" followed by two linebreaks.

Is this a problem with the mime-type (handling of .dll) or the apache
config or is it a problem of the dll-script itself ?

Thank you for your help.
Dom