This abstract explores building a low power music server (50 watts) as opposed to a transport. This is not a one step plug and play solution but it is a great guide to what's available. The PC/Server uses a low consumption power supply/mother board/CPU/Memory combination. Next, reduce the operating speed of the CPU, memory and then disable many Windows features that consume power and create EMI in the computer. This is the total reverse of tweeking a computer to increase it's processing power. Lastly, isolate the PC and DAc from each other by using optical output (not metallic coax).
http://www.audioasylum.com/forums/pcaudio/messages/2/23886.html
http://www.audioasylum.com/forums/pcaudio/messages/2/23886.html