This link:
<a href="javascript:window.close()">Close Window</a>
Will close the window.
Also, you could have the popup window close automaticly after they click to view their messages:
<a href="javascript: void(0)" onclick="window.opener.location='{$config['phpurl']}/viewmessages.php?Cat=$Cat&box=recieved';window.close();">">View Messages</a>
Just a question, are you displaying the popup even when they have no messages? That would be annoying, lol.
