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I use Norton 360. Best all round antivirus s/w i have used so far. It includes antivirus+firewall+phising protection. You can read more about it here: Firewall - Antivirus - Phishing Protection | Norton 360
Norton and Symantec are very popular anti-virus softwares. But check with people using them. they can slow down the computer, sometimes. Some free software are also available, like AVG anti-virus.
Microsoft security essentials ............ Microsoft Security Essentials - Free Antivirus for Windows
