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 an optical out (Toslink)
Apple, Toshiba, Asus and multimedia models of various brands offers this type of output.

If the 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 yourself a SPDIF bracket


Every PC has USB but most audio equipment doesn’t’ have 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: