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.
Allow a program to communicate through Windows Firewall
Turn Windows Firewall on or off
Understanding Windows Firewall settings
Open a port in Windows Firewall
If you’re having trouble allowing other computers to communicate with your computer through Windows Firewall, you can try using the Incoming Connections troubleshooter to automatically find and fix some common problems.
Open the Incoming Connections troubleshooter by clicking the Start button , and then clicking Control Panel. In the search box, type troubleshooter, and then click Troubleshooting. Click View all, and then click Incoming Connections.
For information about advanced firewall troubleshooting, see Windows Firewall with Advanced Security Troubleshooting Guide: Diagnostics and Tools and Troubleshooting Windows Firewall settings in Windows XP Service Pack 2 for advanced users.