If it's Norton 2009 or 2010, the main interface page shows both PC CPU load and a separate meter for Norton itself. Norton 2009 or 2010 runs extremely light on everything I've ran it on or seen it on (very unlike previous editions.)

That said, like many programs, it could bounce the CPU from time to time. But unless something is wrong with the installation (corrupt or something like that), it doesn't peg the CPU and keep it there...