The System Idle Process is just that, an idle process. You can't stop it, you wouldn't want to or the system would freeze. The processor must be kept fed constantly with instructions. It's the OS's job is to keep the processor busy, in NT/2K, the System Idle Process does this.
It says 100% utilization, but it's not smoking your processor, it's really doing nothing and doesn't impact performance.
It lets you know how much space is NOT being used by other processes.