Your computer running more slowly than normal is a sign it may have a virus. Viruses can infect computers by opening emails from unknown people or clicking links in them, plugging in external drives from others, or downloading files from unfamiliar sites without a firewall. Spyware secretly steals users' information and sends it to others, and can infect computers through unfamiliar download sites or not installing Windows Updates. Mozilla Firefox provides the best protection against spyware and viruses compared to other browsers like Internet Explorer.