The internet can be a dangerous place.
Although one’s primary defense against internet badness should, like in the real world, be ones own brain, that is often inadequate due to the cleverness of malware out there. Even so, be smart, stay away from shady websites, and don’t engage in shady behavior.
If you don’t have anti-virus/anti-malware software, particularly if you’re using Windows, please install some. The free Microsoft Security Essentials is an excellent choice and I highly encourage its use. Also, ensure that your automatic updates feature is enabled.
This post is brought to you by the people who get viruses due to their own irresponsibility and then come crying to Yahoo! Answers Computer-Security forum for help and ignore the dozens of previous posts about the identical issue and then post a new question.