The answer must depend on what kind of outside communications you do.
If you just browse major websites (e.g. bbc.co.uk) then you don't need antivirus software, and if you are behind a NAT router then nothing is going to get you from the outside.
If you go to dodgy websites (which includes pilot forums
because they are quite frequently infected) then you need some antivirus software. I use Kaspersky, which has never caused me any problems, over a number of PCs I look after at home and work. Avoid anything from Symantec (Norton).
If you use email then you need to be altogether more careful. Especially if like most people you use Micro$oft email software (Outlook or Outlook Express). Again AV software is a must.
If a teenager has access to your PC, you can forget it. The battle is lost
