<p>I want to disable "Alert window" that I get from login page of one HTTPS site with "untrusted certificate".</p>

<p>ServicePointManager is used for WebRequest/WebResponse:</p>

<blockquote>
<pre><code>&gt; public static bool
&gt; ValidateServerCertificate(object
&gt; sender, X509Certificate certificate,
&gt; X509Chain chain, SslPolicyErrors
&gt; sslPolicyErrors) {
&gt;     return true; }
&gt; 
&gt; ServicePointManager.ServerCertificateValidationCallback
&gt; = new RemoteCertificateValidationCallback(ValidateServerCertificate);
</code></pre>
</blockquote>

<p><strong>but how can I use it with Webbrowser control?</strong></p>

The ServicePointManager is for managed code; WebBrowser is a wrapper around shdocvw, so will almost certainly have a very different programming model.
if
you can automate this (and I'm not sure that you can), I would expect to have to reference the COM version to get the full API (see: AxWebBrowser). WebBrowser only exposes a .NET-friendly subset of the full functionality - enough to get most common jobs done.
One other option might be to get the data yourself (WebClient / WebRequest / etc), and simply push that html into the WebBrowser - but this will mess up external links etc.