Hi Tricia,
I'm not a developer so this workaround won't be as fancy as implementing a lightbox. But, you can use an HTML string in your initial Call to Action link to open up that link in an extra window, which will contain your form. When the user completes the form it should redirect just the second window that was open, while leaving the original landing page as-is.
Try using the code below on your landing page by dropping an HTML block onto your page. Then edit the bold www.marketo.com link to the URL to which you want the window to open - and also change the all caps bold text at the end to say whatever you want the actual displayed/anchor text on the page to be. Hope this helps..
<a href="#" onClick="MyWindow=window.open('http://www.marketo.com','Window','toolbar=no,location=no,directories=no,status=no,menubar=no,scrollbars=no,resizable=yes,width=600,height=600'); return false;">PLACE_YOUR_LINKTEXT_HERE</a>