If I could jump in here? I have a Gigbyte MB that has the Realtek audio card on it. This is a fairly intrusive sound card, constantly fighting for control of the output audio. In my case - and in the case of everyone I know who has the Realtek on their MB - the solution to creating a DAW that swaps the outboard audio device for any and all programs at initiation is to turn Realtek off in the Bios. This is easy to do, open the Bios and find the device: 'Auzula' and turn it off (this is the Realtek soundcard). This will turn off the Realtek device and leave you with only the outboard sound device.
So, like Chris says, find a good outboard device and install it's drivers. At this point you will also need to open Win7 sound device window and choose the new device and the stereo inputs and outputs you wish to be primary for you computer.
BTW, you may think it odd to turn off the onboard sound device when it seems to have some interesting features. Believe me, all of these features will not give you a quality DAW. Using Cubase in the manner it was intended requires a proper external interface device.