This is a classic case of buying surround sound speakers designed for a PC that end up being completely useless for anything else.
Surround Sound Decoders are expensive... the cheapest stand alone decoder I've ever seen is about $150 (if anyone knows of a cheaper one please let me know) and for that price I you might as well just buy some new speakers with a decoder built in. Another option would be to buy a surround sound receiver, which would probably be closer to the $200 range.
the reason is... cheap surround sound systems (for things other than PC use) have built in decoders. The only people who use surround sound decoders are Audiophiles with surround sound systems worth more than my house. So really the only stand alone decoders around cater to those people.
The reason most PC surround sound speakers don't have built in decoding is to save on cost. the decoding can be done with software on the PC and it saves them a few bucks in the speaker system to not include it if they don't have to.
There are MANY MANY MANY cheap surround sound solutions that work well with PCs, Consoles, and whatever home theater device you want to throw at it, in this case a little research could have saved you a lot of money.