Clamav is the most popular free anti virus program for Linux environment.( Of course it scans for widows virus) However, clam is a command line utility and you need some skills for manipulating is properly. There are several graphical front ends for clam av which can make your life easy. The most popular among them are clamtk and Klamav.
Clakmtk uses Perl-tk for GUI. It is available for several distributions. On ubuntu you can install it as
$ sudo apt-get install clamtk
On Karmic it showed up as virus scanner under Applications ->system tools.
On launching it , the following screen pops up
You can select for single user settings or system wide settings and then click save. The main window of clamtk will pop . You can choose either your home folder , a file or a specific directory for scanning.
When the scan is completed a list of detected viruses will be shown. You can select individual infections and right click on them to delete or quarantine.
( My usb stick had nearly 100 infections. I wanted to select them all for deletion . How ever I could not find such an option.)
Klamav is a KDE based front end to clamav. You can install it with
$ sudo apt-get install klamav
It showed up as Applications->System tools ->klamav. On launching it, I got the following screens.
You can select all infections and quarantine them.