Imaginary,
The "Windows Media Sharing" feature of Windows Media Player has to be enabled to allow other network devices, like the Roku or a Squeezebox, to share media files, including music files.
A DAC connected physically via USB does not communicate via a network, so I am pretty certain this would not apply in that case. See if you can find a USB DAC user guide available on-line somewhere. The user guide should indicate what if any special setup is needed for Windows, etc., if any. Most likely, a driver has to be installed and configured and that is it.
Here's an example:
http://www.usbdacs.com/Windows/Windows.html
Interestingly, this indicates that Vista is preffered for better sound quality than XP due to XP resident software that messes with the bits somehow. I can vouch that the sound quality on Vista using the networked media sharing approach is top notch. I suspect the same should be true for a USB DAC + driver as well.
Good luck!