No matter how hard you try to prevent viruses, there is always the chance of one getting through.  That is why it is important to scan your entire computer regularly.  The best way to do this is to schedule daily scans of your computer when the computer is not being used.  If a virus is found, try to use your virus scanner to clean the infected file.  If the file cannot be cleaned, quarantine it until you can determine if the file may be deleted.  If you are unsure what to do with the virus, you may always