Download PCTools iAntiVirus for the Mac. It is free and does a good job.
http://www.iantivirus.com/
About
Introduction
With PC Tools iAntiVirus you are protected against the most nefarious cyber-threats attempting to gain access to your Mac and personal information.
Once your computer has been infected, a virus or worm will usually attempt to spread itself to your friends, family and associates by accessing your email contacts and networked PCs. The infection may also allow hackers to access files on your Mac, use it to launch attacks against other computers and websites or to send mass SPAM email.
PC Tools iAntiVirus provides real-time protection and comprehensive system scanning to ensure your Mac remains safe and virus free.
Protection against Mac specific threats
The iAntiVirus database has been designed from the ground up to detect and remove Mac specific threats. This enables a high level of protection whilst keeping memory footprint and resource usage at a minimum. The iAntiVirus database is not cluttered with signatures for Windows specific threats which your Mac is immune against.
Simple to use
iAntiVirus has been designed with the user in mind. The user interface is purely functional, making it simple to scan for, remove and manage infections.
Features
A variety of scan options
You may perform a variety of scan types using iAntiVirus, this allows you to strike a balance between the time taken to complete a scan, and its thoroughness.
Real-time protection
OnGuard protects your Mac against infections in real time. Whenever an infection is detected and blocked, an alert is displayed below the system menu bar. OnGuard automatically places detected infections in quarantine, works silently in the background and uses minimal system resources.
Quarantine
iAntiVirus quarantines all detected infections, allowing you to easily view and restore items in the case of a false positive (for example: when scanning with engine heuristics set to high).
Automatic Smart Updates
Frequent updates to detect and guard computers against new threats and viruses as well as provide enhancements to iAntiVirus are automatically installed and downloaded through the Smart Update function. Threat signatures are updated within hours of a high risk malware outbreak to protect you from the latest online threats.
Low resource usage
When in monitoring mode iAntiVirus has been designed to work silently in the background, threats are blocked and removed without any system impact, while only a small alert window is displayed to advise you that your Mac has been protected against an attack.
