From Mark Harrison's weblog:

All Windows SharePoint Services customers are entitled to an extended free trial of Antigen for SharePoint. This trial version will be active through June 30, 2006.

To download, simply go to www.sybari.com/wss and fill out the form.

Antigen for SharePoint allows Windows SharePoint Services users to collaborate without the risk of uploading or downloading infected documents or inappropriate content.

The simple and honest fact is that many people who have deployed WSS or SPS don't run any anti-virus software on their SharePoint implementations - and that's a huge mistake. Running plain-ol' AV on the server's file system is exactly the wrong thing to do, because all the SharePoint files are stored in the database where regular AV software can't touch them. And besides that, running real-time AV scans of a SQL database file (which is constantly changing) is a supreme resource and performance killer if there ever was one.

I've worked with Sybari's Antigen products on both SharePoint and Exchange for several years. In my book, it's the best thing in AV-Land since sliced bread. So check it out.