In Drupal 7, you can either use `hook_form_alter()` or `hook_form_<formid>_alter()`, which ever you prefer. Just make sure you get the naming and parameters right. Drupal 6 only supports `hook_form_alter()` however.
When you create these functions, also remember that Drupal may not always pick up on them until you've flushed the cache.
Another important thing to note is that if you want to make changes to the webform fields, you have to make changes in `$form['submitted']`. I made the mistake of originally trying to edit `$form['#node']->webform['components']`, which has no effect.
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Hope that can help.