If you wanted better protection and wanted to use Norton then NIS would have been the better choice but you already purchased NAV2006 so it is too late now. NIS if it is expired will do you no good to have on the computer. The NAV2006 is fine for antivirus but that is all it is so you will need a lot more protection. You need a firewall so if you have XPSP2, at least make sure that is turned on. There are better firewalls than that also, which are free, ZoneAlarm is one of them. You can get that here.

http://www.zonelabs.com/store/content/catalog/products/sku_list_za.jsp?dc=12bms&ctry=US&lang=en&lid=nav_za

You also need anti-spyware and anti-adware protection. These are all free and are a good start.

Spybot S&D (download, check for updates, read the tutorial and scan often, it also does some blocking)
http://www.safer-networking.org/en/home/index.html

SpywareBlaster (a blocker only, download it, check for updates, enable it and leave it alone except for checking for updates occasionally)
http://www.javacoolsoftware.com/spywareblaster.html


SpywareGuard (similiar to SpywareBlaster but works in a different way and does not update as often for that reason.
http://www.javacoolsoftware.com/spywareguard.html

Ad-Aware SE (a scanner, download, check for updates, read the directions and scan.
http://www.lavasoftusa.com/

cwshredder (stand alone unit)(another small scanner for certain things, ALL other windows should be closed)
http://www.intermute.com/spysubtract/cwshredder_download.html