Malware (a portmanteau of "malicious software") is a type of software designed to take over and/ or damage a computer user's operating system, without his or her knowledge or approval. Once installed, it is often very difficult to remove, and depending on the severity of the program installed, its handiwork can range in degree from the slightly annoying (such as unwanted pop up ads while a user is performing regular computing tasks on or offline), to irreparable damage requiring the reformatting of one's hard drive, since much of malware is poorly written. Examples of malware include viruses and trojan horses.