Do you have a sound card?
What motherboard do you have otherwise?
What you use depends on windows really as it doesn't play 5.1 by default any more. Only in movies and games that actually support the features, will it play 5.1.
I used to have those speakers but they died years ago. Bought a pair of Logitech Z906 to replace them - that was over 10 years ago now. They have similar features but they aren't as powerful as yours. I was sad the day I realised my Z5500's weren't working properly. They were actually causing windows to not boot correctly. I had to replace them.
I recently had to swap from analog using RCA cables to digital using spdif cable as my new motherboard only had two audio outputs on the backplate, compared to all previous which had slots for 7.1 support. I could have used the front output for the 3rd cable but that would have been messy.
One spdif cable is cleaner.
If you can do analog do it, but I haven't had any problems running my Z906 5.1 speakers over digital. Only downside I have is the centre speaker may not do a lot any more. My control box lets me run at 4.1 most of the time.
note:
Optical = Digital/spdif
Stereo = Analog (I assume)/RCA cables.
Just to clear up potential misunderstandings about above.