Stopping a for submit with javascript

[eyeRmonkey]

Ok, I have a form that lets you log into the members only section. It is in a frame. I have a system set up so when you click login and you havn't entered anything you get an alert messag that says please enter text. But when I test it and click submit on it gives the alert and then reaload the frame but it takes up the whole page. I also don't want it to have reload either. Oh ya and the script executes when you submit the form.. Any help.