Would it be appropiate to display errors (customer user) via a alert? The reason for this question; in windows development if a user makes a mistake with there login details you would flag a notication using a messagebox in web development what is the custom?

I'm not sure I understand the question? Normally you can just use the .Net Validation controls to say, ensure the Textbox is filled id or a CustomValidator to ensure an email address is valid. If its something like and incorrect password, just send the text to a label that was hidden and make it visible.