Dear mohammad,

Magento uses widgets on registration form. In fact in template register.phtml you can see lines:

This particular widget can be found in template/customer/widget directory. So in order to change select into radio buttons, copy it (template) to your theme and modify, e.g.: 

<div class="input-box">    
<label><?php echo $this->__('Gender'); ?></label>    
<?php $options = Mage::getResourceSingleton('customer/customer')->getAttribute('gender')->getSource()->getAllOptions();?>    
<?php $value = $this->getGender();?>    
<?php foreach ($options as $option):?>   
<input type="radio" name="<?php echo $this->getFieldName('gender')?>" value="<?php echo $option['value'] ?>"<?php if ($option['value'] == $value) echo ' selected="selected"' ?> /><?php echo $option['label'] ?>    <br />    
<?php endforeach;?>
</div>

Hope it help you.

Thanks!.