1. Norton is badly written.
2. Norton has awful rate of false positives - so much in fact that not even VirusTotal uses it (look at
example scan), even though it uses other AV engines of questionable quality, like Zone-Alarm or McAfee.
3. The quality of Norton's coding is awful.
4. Norton's is the best utility if you want to put your CPU under high stress, e.g. to test results of overclocking or to test cooling. That's the only thing it's really good for.
5. Did I mention how awfully coded Norton is?
Please don't use Norton. It's useless. Get a real antivirus like AVG or Comodo. Hell, even McAfee is better than that thing (and McAfee is terrible too;EDIT: Please note I only mean the AV software. John McAfee is a pretty cool dude).