If you're using windows Xp and your windows Firewall happened to be greyed out,as shown below.
Here's an easy trick to enable it back again.
-Start > Run> regedit.exe
-Go to HKLM\Software\Policies\Microsoft\WindowsFirewall key
-Backup/Export that key first, then remove/delete WindowsFirewall branch
-Restart computer
-Windows Firewall should come back again as default