How to Enable Your Firewall

Windows XP

1. Click Start and then Control Panel
If your screen says "Pick a Category", click Switch to Classic View on the left under Control Panel.
2. Double-click Windows Firewall
3. On the General tab under "Windows Firewall", check ON (recommended).

MAC OS 10.3 or later

1. Open System Preferences (located in your dock by default)
2. Click Sharing
3. Click Firewall
4. Click Start

For more information:
Troubleshooting Windows Firewall settings in Windows XP Service Pack 2
MacDevCenter | Mac OS X Firewall Configuration