The expected behaviour on enter in a form seems to be undefined in the HTML 4.01 Specification while the HTML 5 specification says it should be the first submit button in th
If you are using .Net 2 this may help you
...Use the DefaultFocus property to access the control on the form to display as the control with input focus when the HtmlForm control is loaded. Controls that can be selected are displayed with a visual cue indicating that they have the focus. For example, a TextBox control with focus is displayed with the insertion point positioned inside of it. ...