There are some kind of malware (malicious software) that can turn your computer into a bot, When this occurs, your computer can perform automated tasks over the Internet without your knowledge.
Read the full text by Microsoft in this link
http://www.microsoft.com/security/pc-security/botnet.aspx
me also didn't use Antivirus or Internet security software until Oct 2010 last year
until after read the above article about "bots"