I am fans of Kaspersky, but now I change to use Eset Antivirus. Being user of Kaspersky since 2006, I found out that Kaspersky is light and effective, but the newest version 2009 causing conflict to some software I like to use.
For example, when Kaspersky is on, I found that loading PowerISO main page, and right click on virtual drive in My Computer takes about 10 seconds to react. When I pause protection of Kaspersky, all goes fine, take less than 1 second to react. Later I go find forum, and found out that many shares same experience with me. That's not all, I heard that they say Kaspersky also conflict with some other softwares.
For gamer like me, LAN play is very common, so I use Eset Antivirus 4.0 instead of Eset Smart Security 4.0. Eset firewall is weird that it blocks LAN game. For example, if I host Dota, no one can see me, if someone else host, I also cannot see their server. Although the latest build 4.0 firewall has automatic filtering with exception that I set, I feel quite annoying changing around, switching the type of firewall protection. Since this type of firewall (with exception) is new, I did found some bugs, example is it acts like interacting filtering mode when I set it for automatic filtering mode with exception. So, I decided to use Eset Antivirus, with Windows Firewall. Trying all firewall, I feel like I love windows firewall the most, because less annoying, almost no bugs, quite (not noisy like other firewalls), although it may not so efficient compare with commercial firewall.