[DllImport("kernel32.dll", CharSet=CharSet.Auto)]
  public extern static bool CloseHandle(IntPtr handle);

  [DllImport("advapi32.dll", CharSet=CharSet.Auto, SetLastError=true)]
  public extern static bool DuplicateToken(IntPtr ExistingTokenHandle,
   int SECURITY_IMPERSONATION_LEVEL, ref IntPtr DuplicateTokenHandle);

  const int LOGON32_PROVIDER_DEFAULT = 0;
  const int LOGON32_LOGON_INTERACTIVE = 2;

  /// <summary>
  /// This method switches between users...
  /// </summary>
  /// <param name="username">The windows username</param>
  /// <param name="domain">The domain this user belongs to</param>
  /// <param name="password">The users password.</param>
  private void SwitchUser( string username, string domain, string password)
  {
  
   IntPtr tokenHandle = new IntPtr(0);
   // This is for the .NET 1.0 framework...
   // IntPtr duplicateHandle = new IntPtr(0);

   tokenHandle = IntPtr.Zero;
   bool retVal = LogonUser( username,
    domain,
    password,
    LOGON32_LOGON_INTERACTIVE,
    LOGON32_PROVIDER_DEFAULT,
    ref tokenHandle );

   if( !retVal )
   {
    throw new Win32Exception();
   }
   
   // If you use <identity impersonate="true" /> in your web.config,
   // you first need to unimpersonate (you can't impersonate when
   // you're already impersonating...
   System.Security.Principal.WindowsIdentity.Impersonate( IntPtr.Zero );

   // For .NET 1.0 you need to use the DuplicateToken method to
   // create a duplicate handle. In .NET 1.1 you don't need this
   WindowsIdentity newID = new WindowsIdentity(tokenHandle);
   
   WindowsImpersonationContext newUser = newID.Impersonate();
   try {
    // now, do the stuff you need to do with the new user account
    TextBox1.Text = WindowsIdentity.GetCurrent().Name;
   } finally {
    // Go back to the original user.
    newUser.Undo();
    CloseHandle(tokenHandle);

   }

  }