Malware is slang for malicious software. Malware is software designed specifically to disrupt a computer system. A trojan horse, worm or a virus could be classified as Malware. Some advertising software can be malicious in that it can try to re-install itself after you remove it. In the case of invasion of privacy for the purposes of fraud or the theft of identity, software that passively observes the use of a computer is also malware (“spyware”).