I assume you mean that System Idle Process is using that much CPU time. System Idle Process only uses cpu time when nothing else wants it. normally, unless you are playing a 3d game, doing some CG, or running seti, genome@home, rc5, etc, it will be using most of the cpu time.
an (over)simplification of what it does: it asks every other program if they want the CPU, and if not, waits a little, then asks them all again. If you're running MS Office, internet explorer, etc, most of the time, programs will be waiting for you to type something, and until you do, they don't need to spend any time running, so the Idle process gets the time.