1. A method for displaying a window on a user interface of a program, the method comprising: receiving a user selection corresponding to a function of the program, the user selection made from among a plurality of available functions of the program; executing a plug-in module in response to the user selection, the plug-in module comprising computer-executable instructions for implementing the function; receiving from the plug-in module a first document indicating content of the window and functionality available to the user based on the plug-in module, wherein the first document is an extensible mark-up language document; receiving, from the plug-in module, a second document indicating an appearance of the window, the second document being editable by the user to change the appearance of the window after the receiving, wherein the second document is an extensible style-sheet language document; formatting the first document with the second document to create a third document, the third document indicating the content, the functionality and the appearance of the window, wherein the third document is a hypertext mark-up language document; and displaying the window to the user in accordance with the third document, wherein the third document contains markers which the program uses to determine which one of a plurality of windows operated by the program is triggered when a hyperlink is selected by an end user.