I've been using Norton AntiVirus in its last two versions and it has performed faultlessly. On line updates every couple of weeks included in purchase price and it has saved me from virii several times.
Try
www.jungle.com their prices are pretty good, just a word of warning dont be persuaded to buy other Norton Stuff such as Clean Sweep or System Works as I installed these and had all sorts of probs.
Also if you're going on line get a free firewall to protect from hackers.
www.zonelabs.com have a free version of Zonealarm which is v good. If you're not sure about needing a firewall try
www.grc.com and test your computer security.