Slow graphic acceleration issue

hey all, I'm having an issue with my wife's pc.

her mobo recently died, so I purchased a new ASRock motherboard with new DDR3 ram , (4gb of kingston), installed the old hardware in ( HDD, core 2 duo 2ghz e6400 cpu and nvidia geforce 7900 gs ) and then I installed windows 7 64 bit.

Everything non graphic related works like a charm, but all graphical acceleration are choppy. I installed the latest direct x and driver for the card, and nothing happened, I also made sure my old core 2 duo was x64 on the intel website.

games like WoW or diablo 3 are choppy, even facebook games/flash content is choppy. I thought it could have been a dying card so I installed my card in her pc ... same deal. My known working card is choppy in her pc ( I got a zotac 260 gtx, it works just fine in my pc ).

so here I am, with a fresh install of windows, where the graphics are choppy, not sure what to check at this point ? the CPU is not always riding the 100%, the ram is not fully utilized, or the HDD. ressource wise, it seems ok. What gives ?

edit : while writing this, I tried removing all drivers and installing them again, not working.