Viruses and spyware are types of malicious software known as malware that can interfere with computer use, damage files or software, and steal personal information. Malware spreads through users clicking links or downloading files that contain viruses or spyware. Common symptoms of infection include homepage or toolbar changes without consent, frequent pop-up windows, computer slowdowns, and increased junk email. It is important to keep systems and software updated, use antivirus software, and avoid suspicious links or downloads to prevent malware infections.