I have windows 8.1 and I have a memory allocation error and I have to ctrl-alt-del to close the program...

Phys from nvidia wont install either...

Try installing the latest NVIDIA PhysX System Software.

I just tried and it doesnt work.. I have read on some forums that Risen doesnt work on 64 bit system,,,so I m playing it on my old computer

Don't know about 8.1 but Risen runs fine on windows 7 64. Sorry, can't help you with your problem but i doubt it's beeing 64 bit.

I played on win 10 64bit / 980TI lastest nvidia driver

I've installed that

http://www.nvidia.com/object/physx-9.13.1220-driver.html

I run risen in compatibility XP SP3

Into the config file Risen\data\ini\ConfigDefault.xml and modify Fullscreen to false, seem to work for me, i can play it in 4k / 60fps, just need vertical sync, gsync seem to not be efficient on old game

Yep, this helped me. I had issues running it on win 10 but the whole compatiblity and setting fullscreen to false worked like a charm. Thanks a lot:)

I needed to download PhysX software even though I have an AMD card

nvidia.com/en-us/drivers/physx/9_13_1220/physx-9-13-1220-driver/

Remember, AEGIA's PhysX was their own thing, before NVidia bought them out and took their tech.

PhysX and their drivers were needed for certain games during that whole PhysX card era, no matter if you had AMD or Nvidia back in those old days.