One of the prerequisites for installing Citrix XenApp 5.0 on Windows Server 2008 x64 is to install the JRE 1.6.0 Update 5 or later. I downloaded and installed the x64 version of the JRE 1.6.0 Update 7 from http://www.sun.com as directed by the Citrix XenApp 5.0 pop-up message. However, the Citrix XennApp 5.0 installer insisted the correct JRE was not installed. I uninstalled the x64 version of the JRE and installed the 32-bit version. This resolved the issue and Citrix XenApp installed properly.