+1  A: 

That's absolutely wrong.

You should use hook_form_alter() to alter forms. It doesn't belong in the theming layer.

To implement custom validation logic, use a #validate callback.

Wim Leers
yes... i was trying to do on the core level now a days...i have first try on the theme level but it was not getting me the exact result....but thanks 4 replay.....
Nitz
Hey Wim, I have put another question plz check it out.. now i had made module but now i want to add validation...so if you find any error then plz replay me... my question is on.....http://stackoverflow.com/questions/2765193/validation-on-user-registration-form
Nitz