Role Based Security : difference betweenn XP and 2K pro

[JACK]

Hi,

I've got a problem with my asp.net (C#) apps.

Global.asax.cs :

protected void Application_AuthenticateRequest(Object
sender, EventArgs e)
{
if( Context.User.Identity.IsAuthenticated )
{
if( !Context.User.IsInRole(@"SERVER\autorisedGroup") )
{
Response.WriteFile("error.htm");
Response.End();
}
}
}

With Win 2k Pro (with watches) i can see my user's roles
(local groups)
User(System.Security.Principal.WindowsPrincipal)
property m_rolesLoaded is flagged with "true"

This property is flagged to false with XP, (same problem
on Win 2k3 server)

Any idea ?