Well the term antivirus software is very broad. There are hundreds of different antivirus software programs. I recommend you download avast 5 free antivirus.
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If you have any problems just post on my mesage board.
Sure you can! Some antivirus providers even provide firewall with them. If you want a stand-alone firewall, then I recommend going for COMODO Personal Firewall.
Need it - No ... should one have it - Yes. In the present day and age, some people's only interest in the computer is hacking into your system and making your PC (or MAC or whatever) go bonkers....