Okay, I'm still unsure if this is the right section, but here I go!
So, I'm struggling to find a program that can export .prm to .3ds, as I'm trying to edit textures in PS CS5 Extedned, as I heard you can paint on 3ds 3D models in it...
Zmodeler 1.0.7 just wont work for me...
Installing it allways fails, I even tried turning off my virus-protection software.
Is there any other software that can do this? And that is free?
With the help from Jigebren's Import-export plug-in, Blender can do that job for you. It is free software & cross-platform so it should suit your requirements neatly. Jigebren's importer/exporter is not yet released but you can PM Jigebren and sign up as a beta tester. Even though it is beta, I can tell that importing and exporting cars works flawlessly here.
PS: Yes, this would be the right section for such a question I believe.