As I wrote in a tip not too long ago, Microsoft recently released Security Essentials which is their free anti-virus offering. While there are several free anti-virus programs you can choose from, there is a stipulation in most that says you cannot legally install the free product on any machine used for commercial purposes, which would include home offices.

Microsoft Security Essentials, on the other hand, allows for this:

Use. You may install and use any number of copies of the software on your devices in your household for use by people who reside there or for use in your home-based small business.

If you note in the title, the * indicates that while you can use this software on home office machines you cannot use it for corporate/enterprise deployments as this would not fit the terms of the license. If you want something totally free for any environment, the only package I know which fits this bill is the open source ClamAV.

Regardless, this flexibility does separate Security Essentials from the free pack and may be worth further consideration if this is an important item for you.