Never heard of The Shield Pro 2006, it must be new. You going to get varied opinions on this one. Yes Norton is a major resource hog and you computer needs lots of jam to run it.

I would recommend Kaspersky or Bitdefender. I currently use Kaspersky and have been very happy with it and it takes up hardly any resources.

http://www.kaspersky.com/personalpro
http://www.bitdefender.com/