In October, Microsoft released their free antivirus solution:
Microsoft Security Essentials
The jury is still out on its effectiveness in comparison to its peers, but I really like it.
I maintain the network at a small school and I had tried several different “free” antivirus products—they usually were cut-down versions of commercial software, often encumbered with “home use only” licensing.
One key factor for me was the MSE does automatic scheduled updates, but I can also download the file myself and deploy it at my leisure.
I have a cron job that runs at night on a Linux box that brings down the latest virus definition file. Then, when each machine boots in the morning, the file is automatically applied (using wpkg).