As an alternative to proprietary antivirus solutions like Symantec AntiVirus (non-free payware) and Avast AntiVirus (non-free freeware), you might consider ClamAV, which is free open source software. You can get it in Ubuntu by installing the clamav package. If you want a GUI for it, you can install clamtk. You can install packages in the Software Center or using apt-get from the command line as others have suggested (as well as other methods).