Subscription alternatives - Norton Internet Security 2011

I'm currently using a trial version of Norton Internet Security2011  that came preinstalled on my laptop and that is valid for just 1 PC.  Whilst it appears I can click on a 'subscribe' link to renew this subscription, I note that it is much cheaper to go to a high street store and purchase a new  boxed copy of Norton Internet Security 2011 valid for 3PC's.  Can someone tell me what comes in this box and how I then go about putting it onto my existing laptop to extend the protection?  Is this simply a matter of entering a new product key, or would I have to uninstall my trial copy and then reinstall from a CD or download.  Also how would I put it onto a second PC.  Thanks