Desktop Login

I need a small application with following features. 1) It should be plateform independent. ( it should work on Mac, Windows and Linux) 2) After installation it will create an icon on the desktop. First time, On running it will ask for a username and password which will be stored for next time login and open a url and post these username and password. 3) After first time it will not ask for username and password but it will post the username and password to a website URL and open that website which will be logged in with username and password sent by this application and close the desktop application itself (This application work is only to post the data. Website will accept or reject the user itself). 4) There should be an option in program setting to change the username and password. May be user will change the password after login on website but this application will have old username or password so need an update here. You may ask me question if you have any. Happy Bidding.