When to throw an exception?

Question

I have exceptions created for every condition that my application does not expect. UserNameNotValidException, PasswordNotCorrectException etc.

Answer 1

the main reason for avoiding throwing an exception is that there is a lot of overhead involved with throwing an exception.

One thing the article below states is that an exception is for an exceptional conditions and errors.

A wrong user name is not necessarily a program error but a user error...

Here is a decent starting point for exceptions within .NET: http://msdn.microsoft.com/en-us/library/ms229030(VS.80).aspx