I've got an old Soundblaster MP3+ sound card with a SPDIF output. I used a 1/8 inch to RCA adapter, and ran an RCA/coax cable to the coaxial in on my reciever. I had to upgrade the driver for my soundcard and go into a special configuration tool to turn the digital output on. If you are having trouble getting a digital output from your PC, you might see if there's an option to turn it on with the driver.
FYI - I wasn't able to bipass any of the creative "preamp" stuff, like MP3 decoding, EQ, etc., so it really isn't a pure digital signal. My reciever (Harmon Kardon AVR 525) has an onboard MP3 decoder, so I has hoping to use that.
I know it's totally different hardware, but I thought I'd chime in just in case...
Good luck.
a2k