Thanks for all your help. I doubt I can get my money back two years later, and after I've messed about with the card?
Never mind, I had a couple of years of mostly okay use out of it.
I've bought a second hand GT430 1GB on Ebay for £16 including shipping, installed it this morning, and it's working fine. Like I say, I never tried gaming on this PC and for normal dual screen Windows and Linux use, the GT430 is fine.
Now it's running, I can see that I was wrong about the PC. It's got an AMD A8-5500 (4 processors show up in the task manager) running at 3.2GHz with 8GB RAM. Not a powerhouse by modern standards, but still adequate. The AMD has built in Radeon HD graphics, but it's only connected to a single VGA port, so no good for my dual monitors both running at 1920x1080.