Computer running slow?

I have recently updated my security with Norton internet security 2011 and whether its a coincidence but my pc is slow when accesing internet pages.
I am using Explorer and Norton is sometimes popping up with a message about the browser is using high memory (or words to that affect). The pages I view do not contain many images and are just plain standard even when typing emails it hangs and takes an age to load something from my favourites.
Any ideas on whats going on?
Thanks