When only the basic services are running in the background the system ilde process will reach upto the 99-100% mark. That's normal for most systems. However when you have other things running in addition to the system services that percentage will drop. Antivirus and other programs have services running in the background at all times.
The thing to look at is what else is seen with a higher percentage of processor time. Programs minimized in the background will take up some of this right away. If your system has a virus or some type of malware that will take up cpu time while being hidden from the task manager.
