How to open a pop-up window
<p>I am working on a donations website.  The particular web page works by allowing the user to enter a monetary value in a textbox and clicking on the button 'Donate Now' to perform the transaction.</p>

<p>What I would like to do is to bring up a pop-up window with an error message if the user enters an invalid value in the textbox.  How can this be done?</p>
