I advise people to switch to AVG anti-virus. It's from www.grisoft.com. It is free to individuals but companies have to pay a subscription. The advantages of AVG are you get free daily updates. They take about 10 seconds to download each day. It happens automatically when you start up your browser. It is highly effective and scores highly in reviews. It works well with a firewall and scans emails automatically. The biggest advantage is that it is not aggressive and so you will not get crashes, lock outs and slow response time.
In contrast most users find Norton is too agressive in checking everything on your PC and it is costly. You only need an anti-virus to scan when you're on the net, emailing, adding a new program or accessing removable data storage. When you aren't connected, you don't need it.
McAfee performs better than Norton - fewer crashes etc. However you have pay for it.
I also suggest using Ad-aware and Spybot to check for spyware etc. They are both free and very effective and won't crash your system either.
Alphateam