Yep, the quick way is connecting your PC speakers to your audio interface 
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Although windows works fine using several different audio interfaces simultaneously for in and output (windows doesn’t use ASIO drivers), Cubase doesn’t because it can only run 1 ASIO driver at the time. Since your creative ASIO driver can’t access the focusrite, and similarly the focusrite ASIO driver can’t access your onboard soundcard, there is no way to use both in Cubase at the same time.
I suggest you connect your speakers to the Focusrite (and in windows set your default output to the Focusrite). That way you no longer need your creative soundblaster for anything, your audio interface is now in charge of all the in and outputs of your computer.
Note that you should be able to switch the ASIO driver in Cubase without restarting, but it’s still inconvenient.