A great free anti-virus program I use is AVG by Grisoft. You can download it at free.grisoft.com.
I have never had any problems with it and it has saved me from a few viruses. It will automatically update if you tell it to.
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It is a good antivirus – the one problem with it though is that the agreement is that you not in the free license agreement if you use it on a network (which is stupid cause most people use networks of some description), so when I found this out I moved to Avast (www.avast.com) to get my free anti-virus!!!