Killer PC programs are computer applications that are designed to cause harm to a computer system or its user. These programs can be malicious, such as viruses, worms, and Trojans, or they can be legitimate applications that have been modified to cause damage. The term 'killer PC programs' is often used to refer to any type of malicious software that can cause harm to a computer system. The crossword solution for this term is 'apps', which is an abbreviation for 'applications'.