But as shipped, the Windows Firewall offers little outbound protection, and it's not clear how outbound protection can be configured to protect against spyware, Trojans and bots.

Firewalls such as the Windows Firewall work by halting dangerous connections a PC makes over the Internet. The Windows XP firewall offered inbound protection, but did not offer outbound protection. Some malware makes unwanted, invisible outbound connections with hackers, which let them take control of a PC.

In some cases, a computer can be turned into a "zombie" or a "bot," spewing out thousands of pieces of spam over outbound connections without the owner's knowledge.

Competing firewalls such as ZoneAlarm, the Norton Personal Firewall and the McAfee Internet Security Suite offer user-configurable outbound protection, also known as outbound filtering. When Microsoft reworked its firewall for Windows Vista, it added the ability to perform outbound filtering.

But by default, most outbound filtering in the Windows Vista firewall is turned off. In addition, there may be no practical way to use outbound filtering to stop all unwanted outbound connections.