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1. Im disappointed by the governments decision (i.e., I think the decision would affect me negatively, although it might be good for the nation).
2. Im disappointed at the result (i.e., the result does not reflect well the effort).
3. Im disappointed in you (i.e., I believed in you / your ability, but you betrayed my trust).
4. Im disappointed with your performance (i.e., I still believe in you, but your performance was below my expectation).
In standard English Language usually results is used instead of result. We normally say... I am waiting for the results, not I am waiting for the result--may be proper but not suitable.