Adware often manifests through unsolicited advertisements and can degrade system performance by consuming system resources through background processes. Unlike adware, worms spread across networks and primarily focus on replication rather than user disruption. Phishing attempts typically occur through email or other forms of communication to trick users into providing sensitive information rather than causing system performance issues directly. Lastly, Denial-of-Service (DoS) attacks overwhelm services with traffic, which wouldn't explain pop-ups or unknown background processes on a user's device.