Open the file app/design/adminhtml/default/default/template/login.phtml and look for:

<input type="password" id="login" name="login[password]" class="required-entry input-text" value="" /></div>

After this line add:

<div class="input-box input-right" style="padding-left: 5px;"><label for="code"><?php echo Mage::helper('adminhtml')->__('Security Code')  ?>:</label><br /><input type="text" id="code" name="login[code]" class="input-text" value="" style="width:81px;"/></div>

Look for: 

<div class="form-buttons">

Replace with:

<div class="form-buttons" name="no-replace">

Save the file. Done.