For what is is worth, because I am far from an expert in this field, I use ZoneAlarm firewall, anti-virus and anti-spyware. So far, I find it an excellent product.
I abandoned both McAfee and Norton a while ago.
I also use Trend Anti-Spyware 3.
Both of these products have kept me free from the quarterly 'crashes' that used to cause me to reformat my hard drive and reinstall all my software (the result of malware installing themselves on my machine as I browsed the web, as I now know). They are not free, but I do not mind paying when software works.
Steve