How do I modify the "register yourself" form

Hi, I would like to modify the "register yourself" form. I there a way or which template file can do i have to override?

Alternatively is there a hook i can use to make a custom one?

Many thanks

Keith

If you can explain what customizations are you looking to make in the form we can suggest you changes cause its a custom built form, which needs to customize in two to 3 files which produce form, send email and so on.

thanks.