Help - Secure Web Service call fails; The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.

Hello.
I need help in enabling SSL based web service on Windows XP.

I have 2 virtual directories - WS (web service) and Site (client asp.net application calling web service)

I need to make WS secure.
I have installed self-ssl certificate and can navigate to my WS using https.

From my Site I invoke WS method but I get this error:
The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.
What am I doing wrong?
Thank you.


P.S.
I can use event handler delagate to simulate successful validation on the client:
ServicePointManager.ServerCertificateValidationCallback += delegate(object sender, X509Certificate cert, X509Chain chain, SslPolicyErrors errors) { return true; };
But in real lif ...