For those poor guys like me who need to support the guest account on framework 2.x programs (even if the CLR 4 is installed, some old CLR2-compiled programs will still run under CLR2), here is a hacky function that will disable this performance counter initialization issue (see Matt Ellis answer. The problem with his answer is - as stated by some others - it doesn't always work):

    public static bool DisablePerfCountersIfNeeded()
    {
        try
        {
            NetworkInterface.GetIsNetworkAvailable();
            return false;
        }
        catch(UnauthorizedAccessException)
        {
        }

        Type type = typeof(Uri).Assembly.GetType("System.Net.NetworkingPerfCounters");
        FieldInfo fi = type.GetField("initialized", BindingFlags.Static | BindingFlags.NonPublic);
        bool initialized = (bool)fi.GetValue(null);
        fi.SetValue(null, true);
        return true;
    }