This is an answer that PC Magazine provided on this subject:
"You're not the first to worry about the mysterious System Idle Process (Process, not Services). As noted, you can't change its priority. You can try to terminate it, but you'll get an error message. Why? Because it doesn't exist. "System Idle Process" is just the name Task Manager uses to track unused CPU cycles. If your system is only 5 percent busy, you'll see 95 percent CPU usage for System Idle Process. By shutting off everything possible, you're actually cranking that percentage higher and higher. Don't worry! It's nothing—literally."