Windows Vista Security basics - Using Windows Security Center - What's Firewall ?

A firewall can help prevent hackers or malicious software (such as worms) from gaining access to your computer through a network or the Internet. A firewall can also help stop your computer from sending malicious software to other computers.Windows checks if your computer is protected by a software firewall. If the firewall is off, Security Center will display a notification and put a Security Center icon in the notification area. For more information about using a software firewall, see What is a firewall?
To turn on Windows Firewall
1. Open Security Center by clicking the Start button Picture of the Start button, clicking Control Panel, clicking Security, and then clicking Security Center.
2. Click Firewall, and then click Turn on now. Administrator permission required If you are prompted for an administrator password or confirmation, type the password or provide confirmation.
Notes
* If you have a firewall other than Windows firewall, check the information that came with the firewall or go to the manufacturer's website to find out how to turn it on.
* Windows does not detect all firewalls. If you are sure that you have a firewall installed and turned on, you can click Show me my available options to stop receiving notifications from Security Center about your firewall. If you do this, Windows will not monitor your firewall status or alert you if it is off.