hey I just got this surround sound head set with a 5.1 digital decoder laser/coax sounds great with dvd players, xbox and ps3 but I dont know how get my pc to play surround sound any ideas?

I second that. Have the same problem. Here are some symtoms:
I have an Asus P5W DH Deluxe and is using the inbuilt soundcard.
If i switch to Dolby digital 5.1 in the HD-audio manager i get sound via the spdif output (using the RCA). At the same time the sound from the analogue outputs disappears but thats normal and also explained.
However: I'm not getting any surround sound when playing games or watching movies. I do get surround when i try the built in test in the hd-manager (buzzing bee and loopmusic).

cheers

I think that the SPDIF on PC's is used to output a Dolby stream, like 5.1

In order for that to work you need to be running a card that can generate a dolby stream (this is a different thing than passing on the dolby stream from a DVD movie for instance).

I think you might be able to make use of ac3filter (ac3 is code for dolby digital) http://ac3filter.net/ this will multiplex a source into surround sound. I don't think it will help you game in surround over spdif though (it has been some time since I had a DD5.1 decoder, so I don't remember the specifics).

I think it is a pretty dumb setup myself, but there should be an option somewhere in your soundcard panel to output standard sound over the Spdif (I am most familiar with Realtek codecs, as they seem to be on just about every system I build, I always use the latest drivers from http://www.realtek.com.tw/downloads/)

You can get soundcards that makes realtime Dolby Digital sound from stereo sound so that You can plug a cable from the soundcard to Your surround amplifier. Thats how it works isn't it (not sure)?
However for gaming I think the X-Fi is best if Your use headphones as it has good sound positioning in games and EAX 5.0. There are some new cards that might be better.
Sound cards from Auzentech can this realtime stereo to Dolby Digital conversion. Good sound too and Dolby Digital beats everything

Well, if you want to get technical those cards are using software to push the sound out over the SPDIF, as we learned from this creative driver debacle (some guy fixed the Vista drivers, but also was able to open up all features on all older cards, because it was all software based and the new cards were the same.)

So basically it is a function of the drivers for the card, and if you want that support be prepared to pay for it.