There is a legitimate, and vital, process named "System." This process will always have a process ID number of 4. If you have a process named "System" and its process ID number is not 4, then you may have been infected, in which case you should ask for help in the Am I Infected? What Do I Do?
There's another process named "System Idle Process" which is present in all recent versions of Windows. The System Idle Process shows the percent of the time that no program is running, and is there fore an inverse
indicator of total CPU usage. Many people see the System Idle Process taking up 99% of their CPU time and worry that something is wrong, but that's actually how it's supposed to be.