What are the three types of computer viruses?

Types of computer viruses are:
Macro Viruses: Macro Viruses use commands (macros) embedded in other software to infect and spread to other files viewed by that software. E.g. Word and Excel have macros and macro viruses can spread by exploiting these commands..
Worms: Worms duplicate themselves and use communications such as email to spread. They can look at your email address book and send themselves to users in your address book.
File Viruses: File viruses attach themselves to other software. When the software is run, the virus first loads itself into memory so that in can further infect other files or begin damaging the computer.
Trojan Horses: Trojan Horses are programs that claim to perform a particular function but in fact do something different. E.g. they could infect your computer with a virus or erase your files.
Backdoor Trojans: Backdoor Trojans are programs that allow other computer users to remotely control your computer via a local area network or the Internet. . . . Boot Sector Viruses: Boot Sector Viruses are an older type of virus and not so common. They used to infect a computer's startup program so that the virus would become active as soon as the computer started up.
Adware: Is to internet browsing as spam is to email Adware can profile your online surfing and online shopping habits and or placing annoying pop-up adverts, installing additional IE menu helper bars. Often Adware revolves around targeted advertising based upon the web sites you frequent and you may not even be aware that the pop-ups are not coming from the actual web site visited itself but from the Adware software running locally on your machine. Quite often these applications are installed by stealth or by deliberately misleading users to install software that is not required. Spyware: Is potentially a higher threat than Adware as it often collects user details, such as software installed and often sensitive information such as passwords and even credit card details which are then sent to via the internet to a central collection point. Spyware is often installed covertly or by accident via pop-up windows with active-x controls which report that they are doing something benign whilst secretly installing this malicious software.
Page Hijackers: Are applications that redirect links to specific web pages, such as a request to go to a search engine for example, and instead redirect the web browser to a designated address, related to the initial link but often containing advertising or adware. Whilst not as high a threat as Spyware, it is often a sign that your computer has some spyware or adware components installed on it which will undermine its operation.  


If you are asking about types of computer viruses there are File viruses Multipartite Viruses and Macro Viruses