http://www.stickmanlabs.com/lightwindow/

<form id="CaptchaForm" name="CaptchaForm" action="captcha.php" > 
<input type="hidden" name="a" value="submit" id="a">
   <label for="code">Код подтверждения:</label>
<input type="text" id="code" name="code" size="4" maxlength="4">  
<img align="absmiddle" src="captcha.php?a=image">  
<a href="captcha.php" 
params="lightwindow_form=CaptchaForm" 
class="lightwindow_action" rel="submitForm" 
style="line-height: 100%">
<button>Submit</button></a> 
<!--<input type="submit" value="go">--> 
</form>