authentication using Zend_Http_Client

I'm trying to use Zend_Http_Client to make a simple post request on a site that requires authentication. Everything seems to be correct but I'm still getting a You are not authorized to view this page error. Any ideas what the problem might be? I'm certain the username and password are correct

    $client = new \Zend_Http_Client('http://ncmcrm/sales_summary/activity_range.asp');

    $client->setHeaders('WWW-Authenticate', 'Negotiate');

    $client->setParameterPost(array(
            'from_day'  => 1,
            'from_month'   => 1,
            'from_year' => 2012,
            'to_day' => 31,
            'to_month' => 1,
            'to_year' => 2012,
            'user_id' => '{BCDF3313-9DBA-40E7-9CD8-02332F72A64F}'
    ));

    $client->setAuth('******', '*****', \Zend_Http_Client::AUTH_BASIC);

    $response = $client->request('POST');
    print_r($response->getBody());

Here's what I get in Firebug for the POST request that I'm interested in:

Response Headers
Connection  close
Content-Length  4431
Content-Type    text/html
Date    Thu, 08 Mar 2012 19:13:11 GMT
Server  Microsoft-IIS/5.0
WWW-Authenticate    Negotiate NTLM
Request Headers
Accept  text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Encoding gzip, deflate
Accept-Language en-us,en;q=0.5
Connection  keep-alive
Content-Length  120
Content-Type    application/x-www-form-urlencoded; charset=UTF-8
Cookie  ASPSESSIONIDAAQSBCBQ=FEEOKMDAANCMKLGBKDBNKLHE
Host    ncmcrm
Referer http://ncmcrm/sales_summary/sales_summary.asp
User-Agent  Mozilla/5.0 (Windows NT 6.1; WOW64; rv:10.0.2) Gecko/20100101 Firefox/10.0.2

You're setting the WWW-Authenticate header to "Negotiate", but then you're trying to set Basic Auth headers with $this->setAuth.

Assuming the web server will in fact allow basic auth, Removing the $client->setHeaders('WWW-Authenticate', 'Negotiate'); line should work.