update your windows, especially update anything related to net.framework
if still not working, try delete the 'battle.net' folder. If you use XP, go to my computer>>Disk C>> Documents and settings>>all users>>application data (this folder is hidden). There should be a folder called 'Battle.net'. I deleted it then I clicked 'Diablo III Setup' which we used to install the game, 'checking setup files' popped up and this will generate a new 'battle.net' folder for you. You do not need to click install this time, just close it, and run Diablo III on your desktop again. It will start updating files again, but amount of files is less this time, down from 14xxx to 12xxx. After it finishes, you should be able to start the game.
sorry for my English, and I hope this helps