If you are running a windows computer today with out antivirus, you are running with a set of scissors. Yes, I know that with a little bit of care you can keep yourself from being infected. For example using webmail with antivirus (i.e. gmail) and not downloading stray programs off of bit-torrent (especially the “hacked” versions of stuff).
However, accidents do happen. Are you sure that that flash drive that your friend just handed you is clean? Are you sure that that site you are visiting just didn’t get hacked and have a browser hack that will get you?
The best answer is to use great alternative software out there like Linux (my favorite is Ubuntu), however for those who refuse of can’t run linux (there are still some programs out there that are irreplaceable and wine just won’t do everything). It is only responsible to have some protection. This does not mean that you have to go out and spend $100 on the latest copy of Norton however, there are plenty of free solutions (some as in speech and others just as in beer only) out there. They work just as well or even better then the pay software.
The first and my personal favorite is AVG. It just plain works and gets the job done. Next is Avast I have also installed it on some computers that I have repaired for friends. Finally in the “free” corner (free as in speach and beer) is Clamwin. It is NOT a on access scanner so it does not provide the same level of protection as the other options, however for a slower computer nightly scans can at least keep your computer cleaner.