To clarify you have a pair of speakers and you have a single one of the pair connected to your PC? Why?
If you only have a single speaker connected to the PC you aren't even getting stereo.
To get 5.1 you will need 5 speakers plus a subwoofer (that's the .1). If your PC has green, orange, and black audio outputs you would be able to connect powered speakers directly to the PC. Two of those could be the ones you have.