A few things that could make registration easier:

The confirm code states that zero does not exist, I assume this means that the captcha uses "O" instead. Why not just $confirm_code= str_replace('0', 'O', $confirm_code) and remove that comment? It would make it easier on the user.

The timezone should be preselected to the user's current timezone, it's one less field to fill out. You can find the user's timezone with date('e'), it will only work on PHP 5.1.0 or above though, so I guess you'll have to check the version. Not sure about alternatives to this, but it would be nice for boards that are using, and will eventually use PHP 5.1. I don't know if the language select preselects the user's language since I only have one installed.

Lastly, the reset button! It's really annoying having that there; it sucks to misclick the Submit button, hit Reset and have to fill out the form again. I can't think of a time I've ever used it, should it really be there?