Design Specifications and Guidelines - Integrating with the System
Windows 98 and Windows 2000 enable users to display windows across multiple monitors. To properly support multiple monitors, make sure that your application handles an arbitrarily large coordinate space. Unless a user explicitly moves a window to another monitor, display the window on the primary monitor. Similarly, always open secondary windows on the same monitor as their associated primary window, unless the user moves the window to another monitor.
It is also recommended that your application always open its window in the location, size, and position it was in the last time it was opened, unless the task represented by the window always needs to begin in a specific state. In addition, because the user can change the configuration, always check the system settings and adjust your windows to the latest configuration.
Fundamentals of Designing User Interaction
Design Specifications and Guidelines