I use Spysweeper AV on my XP setup (P4 3.2ghz, 1GB mem). It has active spyware/malware/virus/trojan detection and is very light on resources.
Task Manager is showing it using 11k memory and it oscillates between 00 and 02 for cpu usage. I do a lot of music production, so every little resource counts and this one is hardly noticeable when in the background.
The downside is that it takes about an hour and a half to do a complete scan, but YMMV. The scan time doesn't bother me though as it catches everything on the way in, so I never have to scan, and when I do (mainly for low priority spyware), I just leave it running while I'm away.
I don't even use a firewall anymore and have had no problems.
I've tried a few of them on the list, so I'll post my experience with them as it may help you. (btw, I had a computer repair biz a few years ago, so I speak from experience FWIW
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avg free - Probably the best choice for free, but does not actively protect and is rather heavy on the resources. No support.
avast - I briefly tried this one while 'shopping' for an upgrade from avg free. It was easy to use, but it detected far less than the others I tried.
Norton - Total nightmare! It attempts to 'take over' your system with a bulky interface, annoying popups, and very little in the way of customized user settings. Avoid like the plague.
Mcafee - Better than Norton, but I also found it too invasive. It does seem to be pretty good at scanning, but it's "Norton-esque" attributes turn me off.
Trend - Trend has got it together! If I wasn't using Spysweeper, I would be using Trend. It's interface is no nonsense and it gets the job done.
The problem is that no one cure exists. I've had to resort to combinations to clean my clients' computers. In my experience, Spysweeper was the most effective at finding and removing trojans.
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