Digital
A lot of audio equipment has a digital input (optical or coax).
You won’t find many computers with a digital out (SPDIF).
Sometimes the headphone out doubles as a optical out (Toslink)
Apple, Toshiba, Asus and multi media models of various brands offers this type of output.
If your PC doesn't have SPDIF this might a matter of not having a receptacle.
Often there is SPDIF out on the motherboard.
If you don't mind a bit of DIY you can make your own SPDIF out.
http://projects.loetaffe.de/index.php?option=com_content&task=view&id=34&Itemid=12
or get your self a SPDIF bracket
Every PC has USB but very few audio equipment has a USB input.
Slowly but surely the audio world recognize the impact of computer based audio and starts to produce audio equipment with a USB input.
In practice you have two major options to connect your computer digitally to your audio:
- a sound card with a digital out
- an external DAC with USB input