Anti Virus software on managed machines is handled automatically - contact ITHelp@geos if you have a concern about this.
For use on personal or non-managed machines you could install one of several "free" antivirus programs. On Windows our personal experience has shown that Microsoft Security Essentials actually does quite a good job, and is fairly unobtrusive. From Spring 2014 this became the option recommended by University IS.
Systems are also set to perform a "Quick" scan daily at about 13:00.
These changes were approved by University-level committees after a number of recent incidents where malware has been spread within the University, causing substantial disruption.
These changes may lead to some performance degradation, but the impact should be relatively minor except for particularly old/slow systems.