Most mics are pretty sensitive and you dont need to yell into them for it to pick up sound, so im asuming your speakers wouldnt need to be to loud as well.
"I have verified that I can create a sound file that can wake Vista speech recognition, open Windows Explorer, delete the documents folder, and then empty the trash." is what the guy in the article says.
if it can do that i can only image what it can do under and administrator account.
and like he says most people leave there systems on over night so you deff wouldnt hear it them. and most home users are administrators so the whole system would be vulnerable