// Page Level declaration
protected System.Diagnostics.PerformanceCounter cpuCounter;
protected System.Diagnostics.PerformanceCounter ramCounter; 			

// Put into page load
cpuCounter = new System.Diagnostics.PerformanceCounter();
cpuCounter.CategoryName = "Processor";
cpuCounter.CounterName = "% Processor Time";
cpuCounter.InstanceName = "_Total";
ramCounter = new System.Diagnostics.PerformanceCounter("Memory", "Available MBytes"); 

// Call this method every time you need to know the current cpu usage. 
public string getCurrentCpuUsage()
{
      return cpuCounter.NextValue()+"%";
}

// Call this method every time you need to get the amount of the available RAM in Mb 
public string getAvailableRAM()
{
      return ramCounter.NextValue() + "Mb";
}					

// Put this code into button click event
textBox3.Text = getCurrentCpuUsage();
textBox4.Text = getAvailableRAM();