I often get asked, both at work and by friends an family, what to use for anti-virus software? Most people are looking for a free solution but often end up paying for consumer grade AV that they don't really need. My first recomendation is always Microsoft Security Essentials. I've used it for a few years now and while yes, it is a Microsoft product, it really does work! Often, I've found that it works more efficitantly then some of the more beefier products out there. It is also very light weight, often non-obtrusive and honestly it just works.
Checkout Microsoft Security Essentials
For a more consumer/business related solution, Sophos would be my first choice. Their enterprise solution is very easy to use and does so much more then AV. I use it to block users from plugging in external drives, stop them from burning to disks and can even block programs from running on their computers. The GUI is clean and user friendly, but most users won't even notice it's there until they get a warning. I can't say how much it costs as it will vary from company to company, but it is definitly something to look into if you are gearing up for a new enterprise AV solution.
Checkout Sophos.com
