It depends on what you are trying to say. If you mean that the person is crazy or insane on one issue, but usually a normal person, it would be proper to say, "estás loco". If you mean that the person is psychotic or should be institutionalized, it would be proper to say, "eres loco".
The Spanish for 'your welcome', as in 'I am sure your welcome will be a warm one', is 'su bienvenida'. However, I imagine that what you really want to know is the Spanish for 'you're welcome', as in 'Thank you very much' - 'You're welcome' - which is commonly 'de nada'.