Just to elaborate on what people are saying here, many are commenting on it being resource hungry etc is its biggest downfall. However there is a little more to it than that. Possibly one of nortons biggest issues is that its detection rate is actually fairly poor.
In a recent review i saw on a range of antivirus Norton managed a pathetic detection rate of some 68%!
The top award went to Kaspersky with 98%, its without doubt one of if not the best anti virus around but it does cost. If looking for a free one then AVG as mentioned here is quite good as a free one but that got a detection rate of 83% so its still not at the higest end of quality.
Avira got the best rating of the free virus software with 92% detection but comment were made that it tends to nag you a bit to upgrade to the premium package