Quoting: damarrinMaybe try installing fresh 16.04 to a different drive or partition, install the nvidia driver and steam and see if it helps? Looks to me like the installation did not survive the hardware changes as unscathed as it seems at first glance.

Yeah, I'm afraid that I will have to test with another HDD with a clean and fresh install...

Btw, thank you everybody for your help! :)

OK, so after reading @Drak last comment on this topic (problems with NVIDIA drivers on Ubuntu 16.04 / Cinnamon) I've just tried to migrate my girlfriend's PC to the Hardware Enablement Stack with a purge of NVIDIA drivers. Her PC has then rebooted on Nouveau. Good news here as Nouveau seems now able to take care of the GTS450 GPU giving me a proper functional desktop (before the migration to the hwe it only booted on 640x400px which is not that great on a 1920x1080px display). After a purge of Steam installation I was able to install Steam without problem... which is great! :)

The not so great news is that as soon as I install a NVIDIA drivers (for the GTS450 GPU), be it from the official ppa or the "graphical drivers" one, Steam just refuse to launch with the same errors as before. >.<

One more thought: is the system installed in uefi mode or bios mode? I remember nvidia drivers causing trouble in uefi mode, don't know if it's still the case or not. Always worth a shot trying, luckily switching between the two is rather easy.

Quoting: damarrinOne more thought: is the system installed in uefi mode or bios mode? I remember nvidia drivers causing trouble in uefi mode, don't know if it's still the case or not. Always worth a shot trying, luckily switching between the two is rather easy.

Will have a look at that, thanks for the hint! :) (I don't think that it was in UEFI mod as that system has only be installed on old motherboards)