When i put a video card in my pc it doesnt work, but the onboard one does?

The video cards work in other computers but when i put it in mine it doesnt and although the on board vid card works its very slow and choppy when i play games like wow, however it never use to be slow and choppy. Is it my mother board thats crashed?|||You need to disable the onboard video in the bios and also install the driver for the card|||You need to disable onboard video save settings then shut down computer. Then install the video card and make sure you hook the video cable up to the new card.|||i second the above, if fails go to manufactors website for a driver instalation guide