This page talks about computer Viruses, how to detect them, how to prevent them, and has links to webpages with even more and better info than is listed here. A computer virus is a set of unwanted instructions that are executed on your computer resulting in a variety of effects. Viruses can be rather docile, such as simply annoying the user with a sound. These scripts often result in undesired instructions and effects.
Good programmers do a thorough check of their work prior to releasing it to the public.
Viruses are made for a variety of reasons. Some viruses have been created by disgruntled employees that want revenge on a company's product. Others are created to destructively celebrate certain holidays and raise the voice of certain religious and governmental fanatics. Sadly, those that create viruses for the challenge are misdirected individuals making poor use of their skills. If you follow a few simple guidelines, the risk of a virus attack can be reduced to virtually zero, thought there is always a chance of getting a virus if you transfer or dowlnload files from any computer to your own.