Viruses are a major problem. They can easily cost you several hundreds of dollars to repair and hours of lost time.The following links are some of many great articles explaining the most common ways of contracting a Virus and what proactive measures you will need to take to protect yourself.

Email Attachments:
Just like sending packages or mail through a post office, it can contain something that you may or may not want. If you don’t know who it is from, you usually don’t want to open it.

ITG Support Center: virusprotection 

Social Networking Websites:
If you have an account on a Social Networking website, you are establishing your presence on the internet. Your account is a portal for communication.

PCWorld: facebook_virus_turns_your_computer_into_a_zombie.html
eHow: prevent-myspace-from-causing-viruses.html

Peer to Peer File Sharing:
By using an application that relies on other users computers to “share” the file with everyone else, you may receive the file at a very fast download rate, but it does come with its consequences. The problem may not be the application itself. It originates from the website or search engine used to find the file. People may, unknowingly, download these files with viruses attached. Because it is shared with other users, the file quickly spreads and infects all the people downloading that same file which can be masked as a music file, a PDF document, or a compressed file (or executable).

WikiHow: Avoid-Downloading-a-Virus-from-Limewire