Vista - Error calling IIS page configured with windows authentication

The first time that I call an asp page (IIS 6) that uses integrated windows
authentication with a WCF client (generated with vs2008) I get an
authentication error.
If I use a vs2005 client to make the call everything works fine.

I generated a WCF proxy for a webservice that calls an asp page that
requires windows authentication.
Note that, for testing purpose, my asp page does nothing, except from
returning a preformatted soap payload to the caller.

I created a vs 2005 proxy and I succeed on authenticating the request.

MyService a = new MyService();
a.Credentials = System.Net.CredentialCache.DefaultCredentials;
a.Url = "http://MyServer/MyProxyVD/MyProxy.asp";
a.MyMethod(value, ref response);

I tried the same thing with WCF configured as follow:
<security mode="TransportCredentialOnly">
<transport clientCredentialType="Windows"
proxyCredentialType="None"
realm="" />

The response is ok for vs2005 client, instead for WCF I get
The HTTP request is unauthorized with client authentication scheme
'Negotiate'. The authentication header received from the server was
'Negotiate,NTLM'.

I tried changing the credential type only to Windows (Negotiate) o Ntlm
(following http://support.microsoft.com/default.aspx/kb/215383).

Looking (into iis logfile) at the request sent by the WCF client it seems
that IIS authentication is taking place correctly.
I see two 401 reply (as usual).
The third have the correct user (the user that runs the winform), but the
strange thing is that also this response has a 401.5 status code

It seems that the asp isapi is refusing the request.

Having two almost equals clients (one using vs2005 and the other WCF) I
think that it should be some problem on the credential sent by WCF.
My guess is that the WCF credentials are ok to IIS but not for asp.dll.

Modifying the “verify that file exists� on asp.dll I get this error on the
event viewer
So it seemed to me that it is some access denied on some resource…I looked
with procmon but I didn’t see any access denied (either for file or registry).

Strange thing….if I made the first call with vs2005 client, than subsequent
WCF calls are ok.

Any idea on how to configure WCF client for this purpose?

--
Carlo Folini