Once you've connected the cables to the back of the PC, go to control panel > sound > playback devices. Left click 'speakers' once, and then click configure in the lower-left corner of the Window. It's pretty self explanatory from there on.
-what motherboard?
-what is not giving you 5.1 sound?
-do you have the cables connected correct and your windows sound settings set to 5.1?
most music, all windows sounds and most web video will only play out your front speakers because they are only stereo sources. most movies and games however should play out all speakers.