Connecting to the Internet without a firewall is like leaving the keys in your car with the engine running and the doors unlocked while you run into the store. Although you may be able to get in and out beforeanyone notices, someone may take advantage of the opportunity. On the Internet, hackers use malicious code such as viruses, worms and Trojan horses to try to find unprotected computers. A firewall can help protect your computer against these and other security attacks. So what can a hacker do? It depends on the nature of the attack. While some attacks are just nuisancesthat may play simple pranks, others are created with malicious intent. These more severe strains may attempt to delete information from your computer, crash it, or even steal personal information, such as passwords or credit card numbers