The Wild West! A personal computer connected to the Internet without a firewall can be hijacked and added to an Internet outlaw's botnet in just a few minutes.
A firewall can block malware that could otherwise scan your computer for vulnerabilities and then try to break in at a weak point. The only way to make a home computer 100% secure is to turn it off or disconnect it from the Internet. The real issue is how to make one 99.9% secure when it is connected. At a minimum, home computers need to have personal firewall, antivirus and anti-spyware software installed and kept up-to-date. A home network that uses a wired or wireless router with firewall features provides additional protection.