> Does anyone know how to change the default video viewer to Windows Media Player?
You can't really, but you can simply press Shift-Enter (or Shift-Doubleclick) to open the file in whatever player is associated to files of that type in Windows (*).
--> note: If you go to Options | Preferences | Tab:General, there's an option that sets the 'default action' and you can choose between View and Edit. That should be read as 'open in internal viewer' and 'open in external program', respectively. So, if you set it to Edit, the default action (when you just press Enter or you double-click) will be to open the file in your external player.
Alternatively, you can add a button to a toolbar that launches your favorite player with the file under the cursor.
If you want to do that, but first you want to create a separate toolbar for external programs: Go to Options | Customize, and click 'New' on the Toolbars page. Name it External Programs or something like that, press OK and drag the (still empty) toolbar to the desired position. When done, press Close to close the Customize dialog.
You can now add an external program to it (or to any other toolbar) as follows:
Go to File | Open With and choose Add Program. Browse to the desired program (executable) and press Open to select it. You can add a tooltip text and choose an icon if desired. Mark 'accepts multiple files' if appropriate. Then press OK to finish.
Now that the external program has been set up, you can create a button for it. First, open Options | Customize again (as long as this dialog is open, you can customize certain aspects of the underlying program interface). Now, go to File | Open With and drag the external program that you set up earlier onto the toolbar that you made earlier (or to any other one). Then close the Customize dialog, and that's it - you should now be able to select a file and play it by simply pressing the button that you just created.
(*) well, that is the default behavior. What pressing Shift-Enter actually does, is to open the file using whatever method is defined under 'Edit File Using' in the filetype definition of that file. File types are defined under Options | Preferences | Filetypes, or right-click on a file and pick 'Configure Type' from the context menu.
