How does one launch a Modal Window upon arriving at a Web page?

To be exact we have Modal Windows that are based on the LightBox solution, so for example if you click on a link such as this:

<a href="http://www.anoox.com/index_login_form.php" class="lbOn"><img src="/images/social_net_buttons/button_A_32.jpg" border="0" width="32" height="32"></a>

Then the class class="lbOn" will cause http://www.anoox.com/index_login_form.php to open in a Modal Window.

But how does one cause a Modal Window to automatically open upon the User arriving at a Web page?

I assume the answer would be like:
1- Add to the BODY Tag the command onload = showmodal_window_X()

2- But then what to do in the Javascript function showmodal_window_X() to cause xyz.htm file to open via the Modal Window class="lbOn"?