I know I've been pumping out OS X tips for a while now, but I've got something for you XP readers soon enough. This quickie will take a stab at fixing OS X's built in firewall. The firewall may often get its settings changed my other programs which need some port configuration that may ultimately down the firewall. If OS X tells you the firewall is unavailable, this fix may be for you.
Open up a Terminal session via Applications > Utilities > Terminal.
sudo ipfw flush
After the first line is entered you might be asked for an administrative password. This is normal as the sudo command gives you root privileges but must first authorize you. Make sure to press enter after typing the second line. These two commands will tell OS X to rewrite the firewall rules and flush them, or setting the firewall to pre-defined values. A proper execution of the command lines will result in Flushed all rules.
Now, go back to System Preferences > Sharing > Firewall and click Start. You may now close the terminal.