Microsoft touts it as "the anti-annoying, anti-expensive, anti-virus program" and we have to agree.  For small 

businesses (less than 10 PCs), Microsoft Security Essentials is our top anti-virus, anti-malware recommendation.  

The key benefits include:

• It's free.
• Simple to install.
• It's free.
• Easy to use.
• It's free.
• Runs efficiently on your computer without using a lot of memory.
• It's free.
• Virtually maintenance free - updates automatically download once per day, keeping your computer protected from the latest virus and malware threats. 

Check out the product information here and download the newest version (released yesterday) here.  This version is compatible with Windows XP Service Pack 3 (SP3); Windows Vista (Service Pack 1, or Service Pack 2); and Windows 7*

Did we mention that it's free (for small businesses with up to 10 PCs)?