Could not create SSL/TLS secure channel

Hi all,
We have some code that does a post to a https address
using HttpWebResponse. It works fine on server 2003 with iis 5.x or
whatever, but when we try to run it on 2008 with IIS7, it gives us the
following error:
System.Net.WebException: The request was aborted: Could not create SSL/TLS secure channel.
I
suspected it was a certificate issue or something, so i added the
ServicePointManager.ServerCertificateValidationCallback and just
returned "true" but the issue still persists.
this is the relevent code:
        HttpWebRequest Req = ((HttpWebRequest)(HttpWebRequest.Create(("https://secure.[siteaddress]?" + postData))));

        Req.AllowAutoRedirect = true;

        Req.Timeout = 150000;

        Req.ProtocolVersion = HttpVersion.Version11;

        Req.ContentLength = postDa ...