Imp models do not appear when using OpenGL until I kill them, then the models appear as the imp falls down. Everything else is "there", the imp still fires fireballs but is otherwise completely invisible.

Strange, I had no problems before I installed a new NVidia driver for my computer. After that imp models and some other item models disappeared. Troopers and others do appear as usual. Reverting to the previous drivers got all the models back.

(Never mind my comment. After re-installing the driver all models show up. But it was strange that the imp models disappeared just as JoeCapricorn told. Maybe it was a similarish problem. Who knows.)

So, did you uninstall and reinstall the video driver? Or just simply install it over the old driver version? Also, if possible, which driver version were them?