If you do decide that you want to run a virus scanner, then I recommend ClamAV. This is an excellent open source virus scanner, and is available from the Ubuntu repositories. To install it, open the command line and enter the following:
sudo apt-get install clamav
This downloads and installs ClamAV and some of its dependencies. Please note that on many computers, ClamAV will return an error at this stage. If so, just repeat the above command and this should resolve the issue.
ClamAV does not include a graphical front-end by default. You will therefore need to install this separately. Unusually, there are two to choose from - ClamTK for Gnome users, and KlamAV for KDE users. As I'm using Kubuntu, I entered the following:
sudo apt-get install klamav
If you're using the Gnome desktop, enter the following:
sudo apt-get install clamtk
Once it has installed, start it from the menu. Now, you will be walked through a simple setup wizard. From here it's easy to update your virus database and set the areas you want to scan. Please note, however, that you only need to scan the /home directory as this is where any viruses will be found.