To be short: you can not open Outlook calendar from a web application, not from client side, and even less from server side. There were methods that could be used with ActiveX from Internet Explorer, but newer IE versions have closed those security holes. You should not try to solve this by client side means. It won't work.
But you can access the Exchange server (the one that is actually storing the calendar) via the Exchange Web Service from version 2007 on. Look here: http://msdn.microsoft.com/en-us/uc14trainingcourse_5l_topic2.aspx
If you have Exchange version 2003, you still have some possibilities: http://stackoverflow.com/a/878299
If you want to interact with user's calendar, but without the need to connect to the server, you can create iCalendar
] the user could load. iCal is letting you to add, delete or modify calendar entries but with user's interaction.