Have you tried other games on your PC? Did they also cause your PC to restart? The last time I encountered strange issues with a PC game was Battlefield Vietnam. I remember times when the game crashed, blue screened, and returned to windows. I remember visiting the IGN forums filled with users complaining about how such a fun game kept on crashing.
I haven't tried other games on it, but in the shop, they tried, played for a hour or more, I don't know exactly, and it was running perfectly, it didn't restart, crash or anything.
I went on game forum too, but it looks like it works for others,and I also have no problem with it on my old PC. (This PC is whole another story too, we bought it 6 years ago and PC's have changed much since then)
The demonstration in the store doesn't necessarily mean anything. How much a particular game stresses a graphics subsystem depends on a lot of things, such as how it's coded, what settings you've applied, and so on.
OpenGL is an open standard cross-platform API for graphics; DirectX is the Windows-only equivalent.
And it depends a lot on how the game is coded. The game I play that strains my own graphics hardware most (you can tell by watching the GPU temperatures) is an MMO that was written in 2003. It might be making poor use of the card's features, for instance.
Trying the other API might be worth a try. At the very least, it can't hurt.
the it shop may have just used default microsoft drivers for the nvidia card... which would be your problem. i would allow nvidia to detect your video card and download drivers. go to nvidia.com and click support>downloadDrivers>option2. once you have nvidia's drivers installed they will automatically detect future updates and prompt you to update.