Use a Firewall
According to Wikipeida, a firewall is "an integrated collection of security measures designed to prevent unauthorized electronic access to a networked computer system." Both Windows computers and Mac computers come with a firewall that is built into the operating system. There are many different companies that provide software firewalls. It isn't as important as to what firewall is used, as long as you do use one. Often times people will buy new computers that will come with an antivirus package that has a firewall as well. They are usually trial versions that are good for 30 days. After 30 days, they are no longer functional. If you get a new computer, make sure to keep your antivirus software up to date and to keep your firewall enabled!
Windows XP
Watch this video from GetNetWise.org to see how to start the built-in firewall in Windows XP.
Mac OS X
Watch this video from GetNetWise.org to see how to start the built-in firewall on Mac OS X.
Video on Protecting Your Computer with Windows Vista Firewall
