The Workflow and ServiceDesk installer does a few things that UAC would interrupt/interfere with, such as finding all the IIS sites on your machine. (done on the Server Configuration page of the install)
While I have UAC turned off, you MIGHT be able to get around this by running the installer as an admin (right-click, Run As Admin). Also, once the installer runs, UAC can be re-enabled; it's not required to be off when running Designer tools.