There are great resources on the web that provide explanations of what a virus is, how they infect your computer and how to prevent them. Using your firewall appropriately and scanning your system with a competent virus program are simple steps to make sure that your computer does not fall prey to a virus or worm. Here are just a few such resources;
Some viruses can have the appearance of legitimate programs such as Windows Security but continue to "pop-up" and then lead you to a site that offers a "fix" for a fee. Be aware of such scams and use your anti-virus programs to root them out. You too may someday have to "arrest a virus".