Did you get those drivers specifically from nVidia's web site?
Does he by chance have a Mirror Driver from a 3rd party remote desktop control application installed?
Yes, directly from the nVidia website.
I'll check if he has any 3rd party remote desktop control application. Why is that important?
I did find this, and I'm starting to doubt if it will work:
"Photoshop CS5 and Photoshop CS4 can work on 64-bit Windows XP, but Adobe does not support it."
In the meantime he upgraded this weekend to Photoshop CS6, but same story.
Ah. You didn't mention before that the OS was XP.
I do know that Photoshop CS6 does not support GPU operations at all on Windows XP, and I was aware of the "does not support" note for XP x64 from the prior versions of Photoshop.
Sounds like it's time to move up to a modern operating system if the user wants modern features, don't you think?
I did mention it tho :-)
Just a pity that it seemed to be working on XP, but not on XP64.
Yes you did. I missed that - sorry.
In any case, I don't know whether GPU operations were supported in XP x64 with the older versions of Photoshop (CS4, CS5), so I wouldn't have been able to say much. I used to use XP x64 myself, but I was on Vista well before moving up to Photoshop CS4.
Photoshop is reasonably expensive software - at least as expensive as a Windows upgrade... What's stopping the user from moving up to a newer OS?