Seriously, the board is very modern, and there is an SPDIF connector, so I'm almost 100% certain the capability is there. So before you run out and buy more hardware...
What media player are you using? What codecs? If you use Media Player Classic - Home Theatre , and/or the ffdshow codec, there are alternatives in there to just pass on any digital source, eg. DD5.1 or DTS. Or in the case of ffdshow, even expand stereo sources to DD5.1.
Digital output is usually also an output device of its own. Check Sound in the Windows Control Panel if there is one. If there is, you should be able to set what output device to use in most media players. And if you're in XP, check the advanced settings in the mixer.
There are no relevant settings in the Realtek control panel?
However, you won't be able to output the newer "HD" sources, eg. TrueHD and such, through SPDIF. If you want that, you'll need to use an HDMI cable and pass it through the receiver.