Anti-Virus Software
Currently I recommend 'Avast Antivirus' to home users and businesses alike. Avast has several features which make it my favorite solution including the ability to set it up as a screen saver (so you don't have to remember to manually run virus scans once a month) and the ability to schedule a 'boot time' scan. I have always found it to be a reliable scanner but I suggest that you perform a 'minimal' install to avoid some of the strange user interface skins that are otherwise included. Avast has both a free and paid for version and is commercial software.
You can find it here: