Poser 4 depends on the runtime folder being located in the Poser folder. That leads me to believe that a re-install of Poser 4 would be necessary (someone with more indepth knowledge can correct me if I'm wrong here). However, I assume that you have loaded plenty of stuff in your Poser 4 runtime on drive C, so the wisest thing would be to re-install Poser 4 on the D drive, delete the new runtime on the D drive and drag the old runtime over from the C drive. That way you will have an identical Poser 4 installation on the new drive. Poser 5 can access multiple runtimes located anywhere. For Poser 5, you can drag the runtime folder to the D drive and access it from your original Poser 5 installation. The runtime is the folder that takes up the most disk space, so this would help you making space available on the C drive.