As a next step, I would suggest that you install a trial copy of DirectRT 2012 on one of the computers to see if that works. It could very well be that a computer running Windows 7 may not be the most compatible with the 2006 version of DirectRT; have you had problems similar to this using DirectRT 2006 on a machine with Windows 7? You can download a copy and send for the license code as normal, but be sure to ask for a free 21-day trial code in the email to service@empirisoft.com

Also, are the other machines with the same problem also Dell Optiplex 790 computers with Radeon brand video cards?