I used Comodo Internet Security before on my XP. Its firewall protection is excellent, very configurable, but its antivirus engine really sucks. On every routine scheduled scan, there are lots of false positives on well known programs, including Dvd Rebuilder registration key. Also, it scans extremely slowly and slows down my PC a lot.
I uninstalled it and used Norton Internet Security 2009 (and have the automaticac free upgrade to 2010 which I haven’t installed yet), after reading many great reviews over it. Scanning is fast, no false positives, and no resource hog on XP desktop and Vista laptop.
Together with NIS, I also have Threatfire, Malwarebytes, and Spywareblaster installed. I think these should be enough to protect the system. But these are just too much on my very old PC with 512MB RAM, so I just have Avira, Malwarebytes, and Spywareblaster installed.
Avira is great if you can put up with its annoying popups.