Once installed on a computer, a virus may modify or remove application and system files. Some viruses render a computer inoperable; others merely display startling screen messages to unsuspecting users.
Advanced antivirus software programs exist to combat viruses. By definition, antivirus software examines the contents of local hard drives to identify patterns of data called "signatures" that match known viruses. As new viruses are built, antivirus software manufacturers update their signature definitions to match, then deliver these definitions to users via network downloads.