First, forget the computer 'knowing' it has speakers attached, once the card is installed properly, they just work, obviously if they are 5.1 surround or something you will set them up using the audio config to get them how you want them to be.
Next, try this, go to start/control panel/add remove programmes and look for anything associated with your sound card, remove it, go back to device manager, uninstall the sound card, restart the computer, it should find new hardware, go through the motions, if it prompts you to restart the computer make sure you choose to turn it off NOT restart, i had lots of trouble getting my sound working but as soon as i turned it off and switched back on, there was my sound.
You never know.
If that doesn't work, have you looked for a driver on windows update?
Don't forget that it wont show you a driver update in the main window, it will be in a list in the left hand column under 'hardware updates'