Smith has got a point, and if I was rich and had top secret info I would have a combo of Kaspersky, Zone Alarm/Comodo and Spy Sweeper. All I really have though is BitDefender Internet (and I hate it, sorry spiral). I think Kaspersky is much better and much less buggy, but it is too expensive.The best in my opinion is to have separate programs. All it takes is one virus to knock out an entire security center. It takes a more complex virus/trojan to disable many separate programs though. Personally, here's what I'm using.
Avast! Antivirus Home Edition 4 (free)
Spybot: Search and Destroy, MalwareByte's Anti-Malware, Windows Defender
Windows Vista's/Windows 7's UAC
Linux router with NAT/Firewall and content filter
Does great on all of my machines here.