Windows users will have to hold on until next Tuesday (1/10) before Microsoft releases a patch for the much-hyped security vulnerability in Windows Meta Files/Graphics Rendering Engine. Microsoft usually releases updates the second Tuesday of each month. Since this vulnerability uses a weakness in the way Windows displays certain images and since it is a core part of the operating system, it affects all versions of Windows (98, ME, 2000, XP, 2003) and does not depend on which browser you are using. Since the main penetration vector of this exploit is through a maliciously-crafted image, use extra caution when visiting web sites. See Security Now and Microsoft for more information.
UPDATE (1/6/06): Microsoft has now released a security fix for this vulnerability sooner than anticipated. (More information from Microsoft)