If you have ripped to a lossless format though, you can batch-process these files in dBpoweramp which will extract the 20-bit data and encode it to a 24-bit file.
If it does not detect HDCD data, it will not damage the file in any way.
When these files are played back in Media Center, my DAC reports that the audio is 20-bit. (rather than 24-bit)
EDIT: Well that's interesting. I did a test by ripping another HDCD disc, and then processing the files in dBpoweramp.
dBpoweramp has the option of adding 6dB gain during the conversion - I'm assuming that this is part of the HDCD spec, because the disc I tried had peaks of -6.01dB.
Well if you add that 6dB, the file is 24-bit, but my DAC only reports 16-bit during playback.
It only reports 20-bit during playback if you don't add the 6dB during conversion. (the DAC actually analyzes what's being played, rather than simply displaying the format that the computer sends)