If Norton is installed on your PC remove it. Then see if this fixes the problem. Use MS Security Essential or another security software. 

1.Delete Nortan
2. Go to Avast.com
3. Download Avast
4. Free and better protection on installation
4.5. Enjoy a anti virus that does not take the power of some games to run 

 
    Moving to General Software.

The performance of any anti-malware is all relative to the person using the application really. How do you tell if it's using more resources than it should be? Just take a look at the memory footprint of the application itself using either the performance monitor and look at the process tree itself or get process explorer and do the same thing. You'll want to look at the amount of resident memory it is using in relation to other products of the same type. That would mean experimenting or finding a trusted source to look at what was found with their evaluations.
 
Telling if it's doing a better job than some other product would also only make a relative difference as no one single solution will catch all malware being sent in your systems direction. Most will catch the majority (around 90%+) of the known issues out there. What is unknown is how they perform when a new variant which has not been put into the signatures is encountered. How they look into processes of which are vital to the system and are being manipulated by a rogue application or something else. This is where the relative value is necessary.
 
That being said really if you find that the product performs well, doesn't interfere with your gaming and daily tasks then the only difference would necessarily be what you are willing to allow memory, storage and performance wise. If it's all negligible then for one product over another by recommendations it is a moot point.
 
I run a couple of anti-malware products which I have been very happy with. I sometimes take a look at other products to see if they perform as well or equally well but in all it does come down to who you are more comfortable with in protecting your data and information. At least that is my view on this.