I have ZoneAlarm firewall (free edition), AVG AntiVirus (free edition) and also AVG Anti-Spyware and LavaSoft Ad-Aware 2007 (both free).
ZoneAlarm firewall AVG Anti-Virus Free Edition AVG Anti-Spyware LavaSoft Ad-Aware 2007 Do you see a theme here?
I haven't had any problems with viruses, trojan horses, spyware, etc., for a long time (that I am aware of).
I run the anti-virus and anti-spyware at least once every two weeks just to make sure.
ZoneAlarm can be a little annoying in the beginning, with all the warnings about new programs accessing the Internet, but then it slows down significantly and just keeps working in the background, keeping my laptop a little bit safer.
malu