There are five main types of computer viruses and malware: viruses, worms, Trojans, rootkits, and spyware. Viruses need user interaction to spread, while worms can spread automatically by exploiting security vulnerabilities. Trojans masquerade as legitimate programs but harbor hidden payloads, and rootkits give unauthorized access to systems. Spyware stealthily collects user information without permission. The document provides links to free anti-malware software that can help with defense against these threats.