There are a coupl options;

A.) Move the runtime out of the Poser installation folder, before you  uninstall.  This should'nt bee too tough, becasue you would'nt be using any extra space, so as long as you do'nt move it too far it should take too, too, long.  In fact it might only take a second.

B.) Rename it to something unique, and it wo'nt be on the list of folders/files for the Uninstall program to delete.  Then just rename it Runtime, when the Uninstall is complete.