_________ are programmes that affect the effectiveness of a computer system.

A computer virus is a malicious program deliberately written to attach itself to files or programs and spread from one computer to another, often without the user's knowledge. Once active, a virus can slow down the system, corrupt or delete files, consume storage and memory, and generally reduce how effectively the computer performs its normal functions.

This matches the description of "a programme that affects the effectiveness of a computer system," because a virus is itself a self-contained, executable program with the specific purpose of interfering with normal computer operation.

A bug, by contrast, is not a standalone program at all; it is an unintentional error or flaw within the code of a legitimate program, caused by a mistake during development rather than by a deliberately written harmful program. Antivirus software does the opposite of what the question describes, since it is designed to detect and remove threats and therefore protects the effectiveness of a system rather than harming it.

Exam tip: distinguish "bug" (an accidental coding error inside a program) from "virus" (a deliberately created harmful program that spreads and damages performance); only the virus fits the description of a programme designed to affect system effectiveness.