Malicious code can take forms such as a virus, worm or Trojan. It can hide behind an infected web page or disguise itself in a downloadable game, screen saver or email attachment.
Computer viruses are programs that spread or self-replicate. They usually require interaction from someone to be activated. The virus may arrive in an email message as an attachment or be activated by simply opening a message or visiting a malicious web site. Some viruses consume storage space or simply cause unusual screen displays. Others destroy information. If a virus infects your PC, all the information on your hard drive may be lost and/or compromised. Also, a virus in your PC may easily spread to other machines that share the information you access.
Viruses can exhibit many different symptoms. If your computer behaves erratically, employees are advised to contact their organization computer support representative. At home, disconnect the PC from the Internet and run a full virus scan.
Worms are similar to viruses because they self-replicate, however, they do not require any user interaction to be activated. Worms spread because of vulnerabilities or "holes" in software.
Trojans (also known as backdoors) are malicious code hidden in a legitimate program that, when executed, performs some unauthorized activity or function. This can range from stealing your password and credit card information to allowing someone to take control of your computer. To prevent installation of Trojans on your machine:
Denial-of-service attacks are an assault upon a network or web site that floods it with so many additional requests that regular services are either slow or completely interrupted. In some instances, a group of remotely controlled, compromised desktops are combined to jointly attack a target system.
Hoaxes are email messages that resemble chain letters, offer free money, or contain dire warnings and offers that seem to be too good to be true. If you receive a hoax via email, delete it. Sharing hoaxes slows down mail servers and may be a cover for a hidden virus or worm.