Graphics card problems
I had the graphics gtx 460 until it stopped working so i got a 550 ti replacement for the broken one, i tried installing the 550 ti in my computer but the computer didn't recognise the graphics card like it wasn't there at all, the fan was spinning and i definitely had enough power supply to support it, i checked on dxdiag and all it showed up on display was the integrated graphics, i tested it on other computers and the graphics card worked fine and currently i'm using a 9800 gt until i find out why it's not working. i think it may be a motherboard problem since it works on other computers, my motherboard is ASRock N68-GS3 UCC
make sure you have the latest Nvidia driver downloaded and ready to go before hand
uninstall the driver completely, this is best done through Device Manager, right click and select uninstall and then select delete driver software in the window pane.
then insert the 550ti and disconnect from the internet before you power back up, this will make sure windows doesnt try and install a commonly stored driver.
run install of the drivers and it should checkout