where can I find the login block to modify?
Here are the steps for customizing login block available in Drupal.

1) Go to your template.php file for the theme you're working on. Should be in the root of the theme folder for your theme

2) Now check for the below mentioned function in your template.php file. if its not exist then create it and if its exists then append the below mentioned code in the same.

function yourthemenamehere_theme(&amp;$existing, $type, $theme, $path) {
   $hooks['user_login_block'] = array(
     'template' =&gt; 'templates/user-login-block',
     'render element' =&gt; 'form',
   );
   return $hooks;
 }
function yourthemenamehere_preprocess_user_login_block(&amp;$vars) {
  $vars['name'] = render($vars['form']['name']);
  $vars['pass'] = render($vars['form']['pass']);
  $vars['submit'] = render($vars['form']['actions']['submit']);
  $vars['rendered'] = drupal_render_children($vars['form']);
}


Now save and close!

3) Now in side your theme directory there will be a folder called "template". if its not there please create it there only and then create a php file inside that folder called: "user-login-block.tpl.php"
Inside this file, put the following code:

&lt;div id="user-login-block-container"&gt;
  &lt;div id="user-login-block-form-fields"&gt;
    &lt;?php print $name; // Display username ?&gt;
    &lt;?php print $pass; // Display Password ?&gt;
    &lt;?php print $submit; // Display submit ?&gt;
    &lt;?php print $rendered; // Display hidden elements (required for successful login) ?&gt; 
  &lt;/div&gt;
  &lt;div class="links"&gt;
    &lt;a href="/user/register"&gt;Singup&lt;/a&gt; OR &lt;a href="/user/password"&gt;Forgot Password&lt;/a&gt;
  &lt;/div&gt;
&lt;/div&gt;


4) IMPORTANT STEP , navigate to "admin/config/development/performance" and click "clear all caches" -- this is critical because only after deleting cache you can able to see the changes in your login block.

Now you can change the stucture of user-login-block.tpl.php as your wish.

I hope this can help you.