PC World's counterpart in Germany, PC Welt, published a short story today detailing how Windows XP computers running Service Pack 2 may be inadvertently sharing files on their hard drive with more people than they might like.
PC Welt editors Thorsten Eggeling and Andreas Kroschel discovered that a setting created when you install SP2 slightly changes the way the Windows Firewall--one of the new components installed with SP2--creates so-called exceptions to the firewall. The firewall, under normal circumstances, should block all incoming traffic, but exceptions allow certain programs that need to communicate through the firewall to do so.
In this case, the Windows Firewall creates a default exception for the File and Printer Sharing component of Windows XP. If this service was blocked, and you wanted to share files with others on your local network, for instance, the other people wouldn't be able to connect to your PC. However, Windows mistakenly opens the "exception" gap a bit too widely. (The PC Welt story has the technical explanation of the bug.)