I had the same problem when I first installed Skype. I uninstalled an application called Total Recorder and this fixed the problem.
Total Recorder interferes this Skype. There is information on the Skype website about it. (I think Total Recorder presents itself to your PC as another audio source along side your microphone etc. This confuses Skype. Skype doesn't know which source to use.) I don't fully understand the issue, because MSN Messenger and Total Recorder can co-exist quite happily.
Harry
April 2005