// just an example
string categoryName = "Processor";
string counterName = "% Processor Time";
string host = "<name or IP>"; // PRTG's %host
string instanceName = "_Total";

if (PerformanceCounterCategory.Exists(categoryName, host) &&
    PerformanceCounterCategory.CounterExists(counterName, categoryName, host) &&
    PerformanceCounterCategory.InstanceExists(instanceName, categoryName, host))
{
    // despite the above verifications, sometimes a "category doesn't exist", "counter doesn't exist" or "instance doesn't exist" exception still occurs!

    PerformanceCounter counter = string.IsNullOrEmpty(instanceName) ?
        new PerformanceCounter(categoryName, counterName, true) { MachineName = host } :
        new PerformanceCounter(categoryName, counterName, instanceName, host) { ReadOnly = true };

    counter.BeginInit();

    // some counters need the previous value
    counter.NextValue();
    Thread.Sleep(1000);

    // the needed value
    float value = counter.NextValue();

    counter.Close();
    counter.Dispose();
}