Viruses can spread from computer to computer through removable media like disks and drives, or over networks and the internet. Worms can self-replicate without needing to be transferred, while trojans appear harmless but contain malicious code. Common computer viruses include Melissa, Kangen, and I Love You, and antivirus programs like AVG, Norton, and E Scan help detect and eliminate known viruses. It is important to learn about viruses to protect computers through up-to-date antivirus software, file backups, and safe online practices.