the 10/100 means that your motherboard has an onboard (built in) network card which can handle 100mbits (data transfer speed) which is more than enough because on your L package you only transfer at 4mbit. This is for a wired connection though which you wont be using which is why you either need a usb dongle or a card like chicken has recommended above. Whatever you do do NOT buy the one chicken has given you the link for because you will not have a pci express slot on your motherboard. I doubt you'll know how to pop it in either. The best thing you can do from a cost point of view is buy this
http://www.play.com/PC/PCs/-/661/868...archtype=genre and you can stick it in the USB ports on the front of your pc if you have them.
And no, your norton security package wont conflict with your router.