The most effective way to protect your PC from outside attack is to install and activate a firewall program. If your PC is running Windows XP, you already have a firewall program installed on your system. You can make sure that the Windows Firewall is activated by following these steps:
Click the Windows Start button and select Control Panel.
When the Control Panel opens, select Security Center.
When the Security Center opens, select Windows Firewall.
When the Windows Firewall dialog box appears, make sure the On option is checked.
Click the OK button.