My ex girlfriend recently had this exact same problem... I went to have a look at it, but could not find a reason why.
Although, I did try using msconfig (Start > Run > type "msconfig" > hit enter > click the start up tab) and i disabled the tools that were related to the audio (like SoundMAX Manager or something, can't remember the actual name) and it worked.
Like I say, I couldn't find an explanation for this, hence my solution may not work for you (or at all) it may have been coinsidence. I would like to find out if anyone else has had this problem and knows a tried and tested solution...
