https://help.ubuntu.com/community/Antivirus
And then this link for Avast download link to Avast's site, install instructions, how to get free license key, etc.
http://www.ubuntugeek.com/avast-antivirus-for-ubuntu-desktop.html
Note that some of the anti-virus products will just identify a virus and quarantine it.
A Windows virus can run in Linux if you have 'Wine' installed to run some Windows programs.
If you hit a site with one of those fake virus scanners in Linux, you can laugh at it, with it saying you are infected in a Windows machine file system.
But not many virus can attack Linux with it being open source and the OS constantly updated for any virus threat. And it is more fun to make a virus for Windows machines with all it's exploits.