They can vary from annoying, to making your computer completely unusable.
a bomb
It is a destructive virus that rewrites itself and is an executable program with its own code. It usually destroys the program it infects.
A virus is a destructive simple organism which damages its host. The same applies to these programs in their host operating system.
If the program is a virus, it is intended to harm your computer, make your computer a zombi or to gather your personal data from your computer for criminal use.
CIH is also known as the "Chernobyl" virus. It is a destructive computer virus that infects executable files and overwrites critical system data, leading to system crashes and rendering the computer inoperable. It was named after the 1986 Chernobyl nuclear accident.
Destructive computer code is known as malware
Spyware is not destructive like a virus or other malware. It doesn't harm you computer or files, but it may transmit information to places you don''t want it to go.
chernobyl virus
worm
Every computer could have a virus but if you use your computer carefully it can be impossible for the virus to come to your computer.
Generally called a virus, but there are many types. The most destructive are root-kits. These will bury themselves into the programming of the Operating System and from there can affect any part of the system.
There are MANY types of computer viruses. Some may not be so destructive but some can harm or destroy your computer. Viruses can change your settings or steal private information. The worst Viruses can eat/burn you hard drive out and destroy your computer. There are dozens of Virus/Firewall/Spyware programs to prevent this.