Regular firewall can be disabled for various reasons – for example, installing an antivirus with its own firewall. When you uninstall the antivirus, Windows firewall remains disabled and it has to be enabled manually.
To enable the firewall in Windows XP, open "start" – "control Panel" – "Windows Firewall". If the firewall service is disabled, you will be prompted to enable it - answer it in the affirmative. After opening Windows firewall settings select enable and save the changes.
In Windows 7, open "start" – "control Panel" – "System and security" – "Windows Firewall". In the opened tab, select enable network screen save your selection.
Don't forget that the regular Windows firewall does not provide the desired degree of security. It has already been well studied by the hackers found ways to bypass it. Therefore, it is better to use the firewall from the third party turning off normally. When choosing a firewall try also not to take the most popular program. Since these firewalls used by many users, hackers "learn" Trojan horses to get around them. Therefore, the more reliable will be the protection of little-known firewall, perform the basic necessary function, but not allow applications to access the net without your permission, allowing you to create rules and close specific ports.
In that case, if you use the standard Windows firewall, configure it in an optimal way. To learn about network screen for Windows XP, follow this link: Configuring the firewall in Windows 7 is described here: