Personally at the moment I'm using Microsoft Security Essentials (you can download it from softpedia I think), as it has a very low footprint and its really easy to add programs to the Ignore list. After this I might try out the new AVG Beta 9.0 they have out, otherwise, I think it'll be back to Avast, for its low footprint and ease of use compared to programs like Norton, McAfee, BitDefender, Kaspersky.
I recommend avoiding those four, and also avoid PC Tools as they seem to really slow everything down and cause some issues with other programs on my machine. I have yet to try out ESET, as that one seems to be low in memory but still feature rich. I haven't tried Avira or F-Secure because they don't seem to be that good from the table.
In addition to MSE I also have Spybot Search & Destroy with TeaTimer running, and Lavasoft's Ad-Aware, as that captures some bad malwares on occasions in real time. I'd also recommend installing Spyware Blaster for the same passive protection that Spybot gives. I also liked Spyware Terminator at first, they recommend installing ClamWin Antivirus as extraware on install, but didn't eventually like ST very much as it was very naggy.
I used to use ZoneAlarm on all my machines but after the Windows Update KB951748 internet access firewall problem with ZoneAlarm, I didn't really trust it again; plus, it nags far far too much, and makes sharing files on a network near impossible.
I also don't recommend ThreatFire v3 as it really didn't like Vista much, I can only assume the same with Win 7 - Avast was the only real program that had no issues with Vista and Win 7 whatsoever.
Update: Avast has recently caused a bluescreen on a Windows XP machine of mine, caused by one of the security privileges drivers. So am not so sure about Avast from now on.
I also did like AVG very much, however, I don't like its interface and it is very difficult to edit particular components of it. Ad-Aware, on the other hand, does have some advanced editing, with a focus on spyware rather than viruses. It does pop up every now and then so I know it's working - AVG, however, has rarely said anything and so I'm not sure if it's really working.