Computer viruses cause a lot of destruction once they enter into your system. They lead to not only slow processing of the applications but also presence of some anonymous files, corruption of the files and abnormal behavior of the computer. Some of the factors that contribute your computer to get viruses are:

The operating system of your computer: Most of the viruses target Windows based operating systems and if you are using the one on your computer, you are more prone to get viruses than any other operating system.
People who use your computer: A computer which is used by several different people is more prone to receive viruses than a computer that is used by a single person.

Websites that you visit: Some tempting sites like porn sites are the major carriers of viruses and when you visit them, they leave viruses on your system.

Email attachments: You receive a lot of unwanted emails everyday. You should never open these mails, visit their links or download their attachments, even if they are tempting.