A lot of it depends on your habits. My game pc, which is behind the hardware firewall in my router, is running no AV/Anti Spyware/Software firewall, and I have no problems. However, I only go to a few trusted web sites, I use Firefox with Add/Script/Popup blocking add ins, and I don't open unknown e-mails or file attachments on it. I have no problems with the machine.
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However, my web surfing/downloading/family is allowed to touch PC is running:
Zone alarm firewall (I think the free version), more to block anything that trys to phone home than anything incoming.
Ad-Aware, Spybot, and I run Hijack This frequently all for spy/ad-ware.
AVG Antivirus (free), and I also scan it with Trendmicro's Housecall service (free, web based) occasionally as well.
With all that I still occasionally have malware issues, because people click file attachments and an occasional banner or popup add, send and a receive e-mail forwards without using BCC, and surf the web clicking on any link that google brings up that sounds like what they are looking for.
So really, it depends on your person habits. Based on the way you posed you question, I would get everything that has been mentioned, and use Firefox instead of internet explorer. One thing of note: if you do a we search for the products listed, rather than going direct to their sites, you may end up with a lot of similarly named products that are either in-effective, or actually ad-ware laden themselves.
This is the internet... to a non technical user it is about as safe as the white van with a guy offering free candy if you climb in the back...