I don't use any anti-virus software, they often bog down my computer <_<.... however, I do scan for viruses regularly at http://housecall.trendmicro.com (so far, I haven't been hit by any virus :P )

I have used XOOPS before and well, it's nice but it's a bit un-coordinated. Mambo is another option but its user management lacks some features. As for others like PHP-Nuke, e107 etc., the themes all look the same <_<. I still have to try exponent out: http://www.exponentcms.org

So far, I think ...