Ok, so you have ATI X1200. Google tells it has had a similar problem with another Ogre3D based game, looks like the thread didn't end up in a real solution to the issue though
http://www.ogre3d.org/forums/viewtopic.php?f=2&t=67313.
You could run the game in a sort of safemode, worth to try at least. You need to start the game with a parameter "safemode", either on command line by navigating to the installation folder and calling "rwr_game.exe safemode" or by editing the Running with rifles shortcut you use to launch the game normally and making it point to a target like ""C:\Program Files (x86)\Running with rifles\rwr_game.exe" safemode".
The Ogre forum thread suggests that your graphics card might be running low on memory. With luck, it might be just map2 (perhaps with the vehicles) that is going over the limit. You might try another map to launch in to, you'd do that by editing ProgramData\Running with Rifles\settings.xml and altering <map_resource_location>media/maps/map2</map_resource_location>, change map2 to e.g. map4, map4 is the lightest of them in my experience.
The issue in the earlier DrawPrimitive thread you found about jeeps should've been fixed by now. The jeep mesh and rendering was rather unoptimized back then.
I'm not sure how you are going to remove the jeeps; are you going to remove them from the map directly (objects.svg)? I actually don't see another way than that if you can't get to the game to start a new game with vehicles turned off. If you're editing the map, you might want to remove all the vehicle spawn points while you're at it. If you need help I can do that too for you to try.