Hi Melanie63,
You don't mention which platform you are using in your post. However, there was a seperate CD distributed that contained the Full Resolution Preview XT. Rcantin is correct, If you don't have the CD you can download the XTension from the Quark web site at:
Windows: http://www.quark.com/service/desktop/downloads/details.jsp?idx=561Macintosh: http://www.quark.com/service/desktop/downloads/details.jsp?idx=560
Once you have downloaded the XTension you can install it without having to re-install the entire application.