When you remove the old card, boot up into windows with no sound card. I would look in control panel, add remove programs and see if there is any software listed that is for your old card and remove it. It may be that a program for your old card is running and owning the port even when the card isnt present.
Now go into contol panel/system/device manager and make sure everything to do with the old card is not present. In the VIEW at the top, is there an option to show hidden devices?
Now shut down and insert the new card and boot up.
If this fails, try (with the new card in) control panel, system, device manager and remove all the new drivers then reboot so they reinstall.