The free version of Avira by default puts up a splash screen on almost every reboot, and then also regularly shows a temporary notification above the system tray nagging the user to upgrade.
To disable the splash screen: open up regedit, go to HKLM->Software->Microsoft->Windows->CurrentVersion->Run, find the "avgnt" key, and add " /nosplash" to its value.
To disable the nagging: in Windows XP Pro and most versions of Vista, navigate to Avira's program folder, find the "avnotify" executable, and disable Read & Execution in its security settings. If you don't see the security tab in its properties dialog, then go up to the Tools menu for the folder view, choose options, scroll down to the bottom of the advanced options, and disable "simple file sharing". For other versions of Windows (Windows XP Home, some configurations of Vista (I think)), you have to reboot into safe mode to do this. This will not prevent Avira from notifying you of infections or other actual alert events.
We recommend Avira because -- according to extensive testing by http://www.av-comparatives.org/ as well as in our own experience -- Avira is consistently one of the top free antivirus programs in terms of catch rate, while also running only one monitoring process and otherwise having a very very low impact on system performance.
I had to give up using AVG (it was adding several minutes onto my netbook's startup time) so tried Avast. It was okay until I couldn't figure out how to stop it running a full system scan every day that would lock me out of doing anything useful with the machine.
I then tried Comodo but that was an awful resource hog. It really slowed my system down and I couldn't stand to use it. It also quarantined the netbook's touchpad driver as a potential virus without much of a message to tell me what was going on so that didn't make me like it too much.
Avira has been pretty good so far. The splash screens are a minor annoyance but nothing major. I had some problems updating it for a while but that seemed to have been a momentary problem. I'd recommend it.
Oh yeah, I forgot about that -- though I don't know how, it was a serious pain for several months.
Avira's update servers were located in Germany, and would completely fall over every time the continental U.S. decided to update. If your update schedule was done in the middle of the night, it wasn't so bad.
Avira-the-company at first blamed its user's network connections, and then admitted that their update servers were completely overwhelmed. They do seem to have recently fixed this, but only in the last month or so, so we're still waiting to see if it's a long term fix or not.
If this happens to you, you can always run a manual update. Go to http://www.avira.com/en/support/vdf_update.html , download the file, then go to Avira's control panel. I think the menu is labeled "Tools", and "Manual Update" is below that.
Do you bother doubling up with Windows Defender at all? Or just recommend running Avira solo? The reason I ask is that Defender is a bit of a memory hog and I wouldn't be unhappy about getting rid of it.
What about SpywareGuard and SpywareBlaster?
I previously used AVG but found it very annoying. Currently using Defender, SG, SB and then any clean-ups with MBAM.
We don't bother with Defender in XP; Vista's version seems to be a little better, although that might just be an artifact of running on newer machines. I haven't had a chance to play with Windows 7 yet, unfortunately.
Avira by itself has been fairly successful for us (but we also encourage clients to use Firefox+AdBlockPlus, which stops a popular virus vector). The only hiccup we've had was with a new version of "Windows Security 2009" that slipped past Avira for a couple of days and spread by Outlook address books. MalwareBytes didn't find it either, but fortunately it's easy to remove manually.
The little bit that I've worked with SpywareGuard, I've liked it, with the caveat that it can require a lot of user interaction during software installs.
