Windows Xp has a good inbuilt firewall which protects you to some extent from external hacks.
To enable the firewall in windows open the Network connections from control panel.
Then right click the local area connection and select properties.
In the properties dialog box, go to the advanced tab.
Then, under windows firewall, click the settings button.
Under the general tab, select ON option to switch on the windows firewall.
Similarly to disable the windows firewall, you can follow the same steps and select OFF to switch off the windows firewall.